Dice Mechanics

Linear Distribution (Single dice, or unrelated sequential dice)

D20/D30/D%/Polyhedral 

1. Polyhedra: Uses d4-d12, and sometimes d20, more rarely d14, d16, d18. 
2. True20 : D20 used for all rolls Ex: True 20
3. D20 & polyhedra: D20 used to check successes, other dice use for success magnitude (most D20)
4. Roll+X: Any of the above, but add a fixed number to the roll.
5. Criticals: As above, but 1 is always a failure, a 20 is always a success. Ex: DnD 4e
6. D100: Use two D10 to determine a percent score. Ex: GURPS

NON-LINEAR (Dice-sum, dice-pool, and 'fat tails')

DICE-SUM (Normal Distribution)

1. xDx: Roll between 2-5 dice, sum the result. D6 and D10 are by far the most common.
2. xDx+x: As above, but add a fixed number to the roll. Ex: Green Ronin AGE system.
3. xDx+Dx: As above, but add a non-standard die to the roll. Ex: Monsters & Magic uses Roll+dX, with the base roll being 3d6. 
4. xDx&Dx: Sum a pool of dice to determine success or failure, and then roll a second die to determine the degree of success or failure. Ex: Chivalry and Sorcery. 
5. FUDGE: Roll 4d6, sum after using the conversion that 1-2=-1, 3-4=0, 5-6 = 1. ( FUDGE dice are almost necessary)
6. Attributes: Roll 2d6 or 3d6: The median value counts as a zero. The next great value (or pair of greater values for 3d6) counts as a +1, so that that the range is -5 to +5.
7. Flux: Two different colour dice (D6) are rolled, one positive, one negative - the lowest value is taken or if the same =0. Yields results -5 to +5. Ex: Traveler 5
8. Opposed Rolls: Rather than a fixed DC, defense is also rolled, and subtracted from the attack roll to determine success. Ties may be treated as partial successes. Ex: Mayhem

DICEPOOL (Binomial or Negative Binomial Distribution)

1. Success: Roll a number of dice; a die that rolls a certain number or greater counts as a success. D6 and D10 are most common. Typically between half and one third of numbers count as a success. 
1. Count: As per 'Success', but the count of successes matter.
2. Exploding dice: A dice-sum system where the highest value of die triggers the addition of another die of the same type: ie, rolling a 6 on a d6 means another d6 is rolled. 
3. Failure: Every natural 1 has negative consequences.
4. Mixed Pool: A dicepool that contains a variety of different dice.
2. Keep: Roll a number of dice, keep the one with the highest value, treating that as the outcome of the roll.
1. Keep Two: As per Keep, but keep the two highest dice and sum them. 
2. Keep+Count: Take the highest value from a pool, add +1 for other 'success'
3. Keep+Match: Take the highest value from a pool, add +1 for every dice also equal to that value. Ex: Heavy Gear, Pod 9.
4. Wild Dice: Roll two polyhedral dice, one default, one defined by a skill or attribute, and take the higher. Ex: Savage Worlds. 
1. Advantage: Roll 2d20, take the better or the worse. Ex: DnD 5e

ERRATIC DISTRIBUTIONS

1. STEP/RANK: At each rank, the dice rolled changes so as to represent an improvement in the median roll. Ie 1d10->1d12->2d6->d8+d6. Ex: Earthdawn.
2. One Roll Engine (ORE): A dicepool system where values in the pool are grouped into sets; the value is called the 'height', the number in the set is called the 'width'.

"ORE (One Roll Engine) you roll a pool of 2-9 D10 and count matches. The number of matching dice is the width of the roll, and the number of the match is the height. A hard die is always 10, and a wiggle die have any number decided after the dice are rolled. For damage rolls, the width determines amount of damage, and the height determines hit locations. For most generic actions the width is the speed of the action and the height the quality. It is possible to get multiple matches in a single roll, which is how doing multiple actions in one round is handled. " - Jesper Anderson

TN & DC Alternates (Target Numbers, Difficulty Classes)

1. Roll Under: Roll under a fixed attribute, so that low roles are good, bonuses as negative numbers. EX: Advanced Dungeons and Dragons ('Whitebox', 2E) 
2. DC+X: Roll over a fixed attribute, so bonuses are positive numbers. Ex: DnD 3E, 3.5,4, 
3. Margin Triggers: A linear system where beating a DC by a certain number generates an additional effect, often a damage 'critical'. (Mayhem)
4. Risks/Raises: Before rolling, players may reduce their dice pool to obtain additional effects on a successful outcome.
5. TC based on the sum of several attributes

OUTCOMES
With a linear distribution, outcomes are fully random within a certain range; above that range, failure is inevitable. Below that range, success is inevitable. Dice-sum systems generate consistent bell-curves of which the median, minima and maxima are known. Estimating chances of success on a role is more difficult. Reference tables are necessary.  The effect of a +1 bonus is non-linear, but tends to 'auto-balance', as the value of a +1 is greater to the underdog. Dicepool systems where success is binary follow a binomial distribution. Systems that count successes follow a negative binomial distribution. Failure is always possible, although statistically unlikely. Including additional negative effects for 'natural 1;s' can be used to make failure more painful, or more common. Reference tables to estimate success are necessary. With large pools of dice, failures are rare, and large pools may (rarely) generate an awesomely large count of successes.

NOTABLE SYSTEMS

 Travis Casey

The horror RPG "Don't Look Back" uses this: Base roll is 3d6. If you have a bonus, you roll additional d6 equal to the bonus value to your roll, but only take the best three. If you have a penalty, roll additional d6 equal to the penalty value and take the worst three. Bonuses and penalties add together - e.g., a +2 bonus and -3 penalty become a -1 penalty.  Action value is the name for the degree of success. If i succeed by 5 i have a +5 action value. If i fail by 3 i have a -3 action value. In an opposed test, those with the highest positive action value win the contest.

Also note that the "d20/polyhedra" system you have there is used with many other games that don't use a d20 as the basic roll. A lot of old systems use a separate "damage die" in combat. A similar concept is also seen in 3rd edition Chivalry & Sorcery: you roll 2d10 to determine success/failure, and an additional d10 of a different color, which determines degree of success/failure. Green Ronin's "AGE" system uses 3d6, one of which is a different color. All three are added together, along with an attribute bonus, but one of the three is special and generates "stunt points" that can be used if you succeed.

Bonus Sizing

Travis Casey

September 29, 2015

A few recent OSR games, such as Swords & Wizardry White Box, Delving Deeper, and Bloody Basic, have adopted OD&D's attribute modifiers for rolls. Those are: 8 or less: -1;  9 to 12: 0; 13 or more.

On the positive side, I like the fact that this divides up abilities into "high-middle-low". You don't get the effect from later versions, where many players seem to feel that unless their primary attribute for their class is quite high, they're "hopeless". On the negative, though, the modifiers feel too small. These games use variants on the d20 System, so a +1 is only a 5% change in the chance of success. It's such a small bonus as to be rarely useful. 

 +1 OD&D intended those modifiers to be used with a 2d6 system, not a 1d20 one. A -1 or +1 is much more significant on 2d6 than with 1d20. So, I'm suggesting that if people want to use this style, but also want to use the 1d20 resolution mechanic instead of the original 2d6, they use -2/0/+2 as the modifiers instead. I'm suggesting doubling the numbers to give a statistical impact closer to what those modifiers had in the original system.

Original D&D was meant to use the Chainmail combat system... which used 2d6. In the context of a 2d6 system, a +1 is much more significant. If you normally need a 12 to succeed, it makes you 1/18, or 5.55...%, more likely to succeed. In the best-case scenario, if you needed an 8 before the modifier, it makes you 1/6, or 16.66...% more likely to succeed. Thus, a +1 in original D&D is more akin to a +2 when you're using 1d20.  

Gygax quickly changed the modifiers, doing so in Greyhawk, the first major revision to OD&D. My guess is that he did so upon realizing that since D&D was far outselling Chainmail, most players were using the 1d20-based "alternative combat system" rather than the originally intended system. The upshot is - if you're contemplating making a system with an OD&D-style flat modifier, but are using a d20-based system, I'd suggest making the modifier be -2/+2, instead of -1/+1. That will keep things closer to the original intended effect of the modifiers. 

...I'm not talking 'double all steps, so you go +2, +4, etc". I'm talking about three tiers, period: -2, 0, +2. Nobody would have a +8 from a stat, unless you're dealing with a girdle of giant strength or the like, in which case it's really a magic item bonus... 

...The really nice thing about not gradating it like that, is that it then doesn't cause a drive toward higher stats. On 3d6, the chance of a 13 or better is 26%. Thus, with six stats, the majority of characters (about 5 out of 6) will have at least one... 

Alexander Staniforth 

I think there'd be something to be said for a little more nuance. -2/-1/0/+1/+2. That way ridiculously high stats DO matter more, as do ridiculously low stats while at the same time it's still a relatively simple gradation.

Paul Goldstone 

Bigger bonuses skew balance, especially at higher "levels". Lower bonuses also allow other "magic item bonuses" to have relevance. A +1 magic sword is irrelevant if a 18 stat gives +8 to a roll. I think the issue here is on a (3-18) system fundamentally they have made top end and low end stats irrelevant, there seems to be no (game mechanic) difference between 13 and 18. Perhaps a "roleplay aspect", but game mechanic wise irrelevant.

Attack Role Complexity

Matt Miller

The more complex combat is, the more time it takes, the more it becomes the focus of the system.Would it be possible to have three distinct steps in an attack? Mayhem  coupled the two together (one step) so that the margin of hit resulted in more damage dice.

Rob Hicks 

In Mayhem, the roll to hit and the roll for damage were two separate things, but they were interconnected based on the "crit" range, the number by which it the die hit. It worked fairly well, but I was always eyeing it wondering if I couldn't streamline things somehow.

Matt Miller

D20 has two steps for damage: Roll to hit, and then and (independent) roll for damage. My object is to get the final damage number down as low as possible.

What about...a roll to hit, a damage roll, and an armor roll? Roll 1d20 (vs AC), then roll damage (vs. Armor). Rolling damage vs armor is very different from Armor subtracting from damage. Subtracting armor from damage generates 'false hits', where your attacks succeeds...but does not damage.

Players dislike those. But players like critical hits. So the trick is to make the damage roll translate into a count of criticals, with values of 1, 2, or 3. (Call them 'wounds') And that...I am not sure how to do. Not in a balanced, mathematically simple way. DMG minus armr causes 'false hits', while damage/armor is mathematically complex. Presume our damage dice will be regular polygons (d4-d12) with some use of multiple dice.

 A ranked table might work, where >0, >10 and >20 determine if its 1, 2, or 3 wounds. But the 'margin' number would vary by armor, not weapon..

Tom Vogt 

There are many dice mechanics that accomplish exactly this. Cortex Plus has a system where you roll 3d6, take the highest two for the success (hit) and the low one for damage. Or Godlike with its height+width, it evolved into the aptly named One-Roll-Engine. As soon as you look outside the boring mainstream games, you find a lot of very interesting and innovative game mechanics. I'm working on another system right now that will also have one roll to resolve both hit and damage. It's by no means a first

Travis Casey 

Quite a few older games have a roll to hit, a hit location roll, and a damage roll. Some throw in other rolls as well. BRP, in the complex case, has roll to hit, roll for the opponent's parry or dodge, roll for damage, and roll for hit location.On the opposite end, you have systems like Torg, where there is no separate roll for damage - the margin of success of the attack roll is instead added to a weapon damage value to give the damage done. Thus, each attack requires only one roll.

Paul Goldstone 

Rolemaster accomplished a lot with one roll (plus possible crit) which works nice, though a lot of tables.... I suppose there is a need to consider how important or how much of a focus is combat for the game, as well as whether the skill system will follow the same rolling premise. Finally how long it takes to work out and complete every roll. Played a few games where a "quick combat" takes over an hour for a couple of rounds for 4 players and 6 opponents can be a bit much. Finally how deadly/realistic is the combat system intending to be which is another important factor.

Rob Hicks

When I was working on Blueshift, that one used a single attack roll to speed things up, but that was also because I wanted to do automatic gunfire, which meant multiple combat rolls simultaneously, and I didn't want to have to roll multiple damage damage dice separately for each attack, and didn't want to create a huge variability in possible results by counting up lots of damage dice. If course, the other option involved multiplication, so go figure.

Travis Casey 

My own homebrew uses multiplication for damage: the amount you hit by is multiplied by a damage factor for the weapon used, with the adjustment that a zero (just barely hit) is multiplied by 1/2. The numbers involved are usually small - less than 10, so they're in the range of the multiplication tables most people have memorized.

Jesper Anderson
 Roll to hit, roll hit location, roll damage, depending on armor at hit location roll glancing (to further reduce damage). That's essentially how Phoenix Command Hand To Hand does it.

Attribute Score Distributions and Associated Bonuses

Robert J. Grady

Small plusses is one of the strengths of OSR games, as it keeps expected target values within a certain range. That said, basic D&D went up to +3, which is a nice range.

Alexander Staniforth

I think there'd be something to be said for a little more nuance. -2/-1/0/+1/+2.That way ridiculously high stats DO matter more, as do ridiculously low stats while at the same time it's still a relatively simple gradation.

Paul Goldstone
Bigger bonuses skew balance, especially at higher "levels". Lower bonuses also allow other "magic item bonuses" to have relevance. A +1 magic sword is irrelevant if a 18 stat gives +8 to a roll

Travis Casey

I'm not talking 'double all steps, so you go +2, +4, etc". I'm talking about three tiers, period: -2, 0, +2. Nobody would have a +8 from a stat, unless you're dealing with a girdle of giant strength or the like, in which case it's really a magic item bonus. A few recent OSR games, such as Swords & Wizardry White Box, Delving Deeper, and Bloody Basic, have adopted OD&D's attribute modifiers for rolls. Those are:

• 8 or less: -1
• 9 to 12: 0
• 13 or more: +1

What I'm pointing out is that OD&D intended those modifiers to be used with a 2d6 system, not a 1d20 one. A -1 or +1 is much more significant on 2d6 than with 1d20. So, I'm suggesting that if people want to use this style, but also want to use the 1d20 resolution mechanic instead of the original 2d6, they use -2/0/+2 as the modifiers instead. I'm suggesting doubling the numbers to give a statistical impact closer to what those modifiers had in the original system.

Matt Miller

The varied gradiation provides a mechanism to make characters more directly comparable. A first level wizard with an 18 is better than one with a 12. At some point, players are unwilling to keep lousy characters, and just re-roll until they get something acceptable. 

Paul Goldstone

I think the issue here is on a (3-18) system fundamentally they have made top end and low end stats irrelevant, there seems to be no (game mechanic) difference between 13 and 18. Perhaps a "roleplay aspect", but game mechanic wise irrelevant. There must be a reason, perhaps its all about reduction of the munch (min/max) players, but I would assume, having limited knowledge that f the games you have highlighted, that the game designers had reason for it all in this specific instance

Alexander Staniforth

That's why I'd suggest a -2/-1/0/+1/+2 approach. A point to higher stats without making them ridiculous and crucial to high-level characters. The really nice thing about not gradating it like that, is that it then doesn't cause a drive toward higher stats.On 3d6, the chance of a 13 or better is 26%. Thus, with six stats, the majority of characters (about 5 out of 6) will have at least one. I have to admit, I like luck in character gen (though I also like point-buys... either is fine by me, depending on the game) and I'm not too obsessed with games being balanced.

ORE (One Role Engine)

Matt Miller:
Uses d10, could likely use d6. Biggest problem I can see is the dicepool problem, where you've got 10 dice, arranging them into sets is time-consuming. Not sure I understand 'Hard Dice'. They act sort of like criticals, making a successful roll even more successful? Expert dice are clear--pick a number when you get them--seems highly suitable for 'specialist' abilities. Wiggle/Trump/Master dice boost the...'width' of any set, making the action faster, but not necessarily more successful...

Jesper Anderson: A hard die is always 10. So if you have two hard dice and four normal dice, you roll 4d. You are ensured to always have one set of 2 10's in that case, but it could be wider, or you could have more sets. Wiggle dice are really powerful. You get to pick their value after you have rolled. If you have only one wd it will ensure you get at least one success. Arranging in sets is not time-consuming. It's pretty much instant, even with a 9d pool.

Travis Casey: I've had Godlike since it came out, but never gotten to play it. I have, however, run a few sessions of A Dirty World, which uses the same dicing system. We found that quite often, rolls gave no matches at all. Part of this is probably because ADW is meant to be a gritty, low-powered game, and quite often, my players were rolling 4 dice or fewer. This also meant that the fun features of the system, where you get multiple sets and have to choose how to allocate them, almost never came up. If you're going to use the system, I recommend making sure that attributes are assigned in such a way that players tend to have fairly large dice pools, in the 6+ dice range.
Jesper Anderson 
... or have wd and hd. That makes a difference!

Travis Casey: Yeah, I'm sure it would have. ADW doesn't use those aspects of the system, though. I might do some thinking about how to incorporate them, for when I use the system again. Since it *is* low-powered, I as GM didn't really have a problem with the constant "no success" - to me, it meant the PCs just hadn't accomplished their goal yet. A few of the players were noticeably frustrated, though, so I had to reassure them that no match doesn't mean failure when you're rolling against an opponent.We only got to play three sessions or so, and by the third one, I think they were settling into the system comfortably. It did take an "adjustment period", though.

Jesper Anderson Yes, with 4d you will get a match half the time or so. It's how the system works. Even in Godlike most rolls are with low pools, and the attrition rate tends to be very high. ORE as a whole takes an adjustment period. My present group has not tried it, and they have a firm opposition to it. Which is rather strange, considering how many weird systems we've successfully run.

Scott Rhymer 
I've been playing in a Wild Talents game, and while the mechanics work adequately, the wiggle/hard/other dice was -- IMO, so take that as you will -- overly complex. We all found character creation a hot mess. The general consensus for our group is that, in trying to make things simple and fast, it paradoxically introduced more complexity than it probably needed to.

Travis Casey 
I do recommend checking out the "A Dirty World" version - it's very different from Wild Talents/Godlike, with only the resolution system being shared. Character creation is totally different, and much simpler... and what you do with the outputs of resolution is also very different. From a game design point of view, ADW vs Godlike is an excellent example of how different two RPGs can be while sharing the same resolution system.

Paul Mitchener 
I've run a lot of Wild Talents and Reign. The One Roll Engine is one of my favourite mechanics.
And the Wild Talents settings (I'm thinking of Kerberos Club and Progenitor especially) are fab. Though the writer of Blood of the Gods is a complete jerk, so that's better missed.

"everything the designer adds builds in some sort of genre assumption"

Travis Casey 

.....games with logarithmic ability ratings tend to scale up and down better than those with linear scales, making them suitable for a broader range of genres without major adjustments. As another example, in a skill-based system, adding a single new skill can allow you to support multiple new character types, whereas in a strict class-based system, each new type requires creating another class.
     Thus, while everything the designer adds builds in some sort of genre assumption, a designer can attempt to make choices that restrict the range of genres minimally. Perhaps just as important, the designer can also explain the choices that were made, why they were made, and alternatives... thus creating a 'hacker's guide" for those interested in using the core of the system for other genres. Some games don't attempt this at all, while others make it a major selling point.

Genre and Setting

Jeremy Forbing

Genre and setting are two different things....every role-playing game simulates a genre, whether it has a setting or not. A decision like how many wounds can a character take before being killed is a genre decision. ....There are genre assumptions baked into the design of Fate and ORE.
    Lovecraftian investigation is a perfect example. The Call of Cthulhu system, and the Gumshoe system as well, are designed to prioritize and allow a slow trickle of revealed information that quickens into a flood towards the end that breaks characters' minds, thus emulating the feel, pace, and convention of Lovecraft's major short stories. A Fate version of a Lovecraft story would feel much pulpier, and likely have a much harder time that slow burn over a long campaign. This isn't a knock against Fate, a system a like which is designed for fast-paced story-driven adventure.
    Sure, you can run any genre with any system, but will it feel like a particular kind of story from that genre? Unlikely, unless the system is designed for it. D&D in 1979 and D&D in 2016 are both generic fantasy systems, yet both have assumptions about fantasy built in (magic from Jack Vance, Poul Anderson paladins, Tolkien races), and in many ways they were designed with different genres of fantasy in mind-- for example character death lurked around under every corner in the 70's, now it is rare.
    World War II RPGs where you keep track of ammo vs. ones where you don't feel like very different genres of war movie or novel. System is always built on ideas about the general tropes that create and constrain the kind of stories that will be told with it: in a word, genre. 

Travis Casey 

Genre and setting are strongly interlinked.....As far as all games having a genre "baked in" - we're talking about *systems*, not games. Different games using the same system can alter various parameters. For example, D&D 3E, Star Wars d20, and Mutants & Masterminds all use the d20 System... but they all use different subsystems for determining the effects of damage. Quite a few "universal games" offer multiple subsystems for various things, to allow "tweaking" the game to fit the genre you wish.

Jeremy Forbing 

I think the distinctions between game and system are even more nebulous than that. When you talk about the D20 system being one system, for example, it seems like you're really talking more about one resolution mechanic. Even single mechanics within systems have genre considerations,....
     Still, some of those games run into trouble when trying to step outside the original assumptions of the stories that would be told with it. M&M replaces the damage system with one much better suited to cape types, but still has some baggage built in from an assumed hierarchy of levels and internal game balance. It also sometimes chokes when applying its basic mathematical model (essentially, opposed ticks of 5% up and down a spectrum of possibilities of success of failure) to the dynamics of superheroics.
     The same designer (Steve Kenson) wrote both M&M and a superhero version of Fate, and I would argue Fate's mechanics suit the story driven flow of comic book stories way better, even though the same gifted writer with a deep knowledge of the genre worked on both.
     ...all D&D type ... games.... assumes many things beyond Hit Points and the likelihood of death; it assumes heroes who grow in formidability across a large scale over time ....tracking the acquisition and use of individual wealth and possessions....regular interaction with contained environments that want to kill you....characters that receive ambient information with varying degrees of opacity
     You can design a role-playing game without making assumptions about setting beyond the general (gravity existing, etc.), but not without building in certain givens about genre, even if you do so unconsciously or in an attempt at realism. 

Platform Systems

Matt Miller

GURPS, FATE, ORE, BRP, Hero System, and Savage Worlds are all 'systems', in the sense that they provide a resolution mechanic that can be applied in a wide variety of contexts. They also have the 'core' elements of a system: Primary Attributes, some way of using skills, and some way of dealing with abilities. But while they provide the skeleton, it's a skeleton that requires a lot of fleshing out by the GM. I call these 'Platform' systems.
I contrast this with other games where there is a substantial amount of game content tied to a specific implementation of a game, and is often closely coupled with a specific setting. Examples of which being Monte Cook's '9th World', or the '13th Age' system or Dresden Files.
Examining DnD 5E, with its extensive lists of overlapping classes, lack of specific geography and early release of OGL/SRD, I begin to wonder if WotC/Hasbro isn't trying to make DnD into a 'platform' system.

Keith J Davies

And contrariwise, 13th Age can be played with other than their published setting -- it's more work, because they've hung so much off the icons. You'll want to replace the icons, but once you've done that you're most of the way there.

Marco Subias

...GURPS, FATE, ORE, BRP, Hero, are all at least somewhat generic systems with supplements that support campaign specialization. I can use any of those systems right out of the box to run all sorts of campaigns involving all sorts of time periods, tech levels, genres, and settings. DnD? Not so much. 

 Travis Casey

A few thoughts here:
      First, how RPGs advertise themselves doesn't always match their reality. Hypothetically, for example, The Dresden Files RPG is setting-specific... but it's flexible enough that it works quite well as a generic urban fantasy RPG. As a more extreme version, The Fantasy Trip is supposedly set in the world of Cidri... but we're told very little about Cidri, and it's explicitly made to be a huge world where "every GM's campaign setting can fit". Thus, TFT is really a fairly generic fantasy game.
      Games that actually tie the setting and mechanics strongly together are really fairly rare. Nobilis comes to mind as one, as do Everway and A Dirty World. Most games become "setting specific" not because of the actual mechanics of play, but because of the built-in details about the game world. D&D is a classic example: what's ostensibly a generic fantasy game comes hard-wired with a huge number of assumptions about the game world built into its classes, races, and monsters.
      Second, "universal" vs "setting specific" isn't a binary switch; it's a continuum. Even ostensibly "universal" games tend to have certain things they handle better than others. For GURPS, it's fairly low-powered, high-realism worlds. HERO does well in genres where its power-building system can shine, such as superheroes and fantasy. On the opposite end of things, we can say, for example, that 'Chivalry and Sorcery' is much more specific in the range of game worlds that it's designed for than The Fantasy Trip is, even though both are meant for "medieval fantasy"
     Third, people have differing ideas of what "game system" means. For example, A Dirty World is hypothetically an ORE system game, but it actually shares almost nothing but the die mechanic it uses with the other ORE games. D&D 3E and d20 Modern are both "d20 System" games, but they have completely different class setups. Going further, Mutants & Masterminds is also based on the d20 System... but it completely drops the idea of classes, and uses levels simply as a convenient measure of power (using them to set standard power point totals and power limits).
     In the last year, there's been a lot of "White Box" based systems released - White Star for SF, Warriors of the Red Planet for sword-and-planet science fantasy, White Lies for espionage. They all share the class/level paradigm and the same core mechanics, but they each have their own set of classes, races, and other supporting rules for their genres. This is really just a commercializing of something people have been doing since RPGs were new - taking whatever system they had or liked, and adapting it to the genre(s) they wanted to play in.