The broadest principle is the concept of Mastery. I'd link to Carthain's original page on revised grappling rules, but they seem to be lost. I will quote from a Google cache below, but quote the relevant principle more visibly.
Carthain said:
The d20 system was created originally with an eye towards the concept of mastery. Mastery is the idea that someone can obtain knowledge over a wide area of the game by learning one area. As an example, someone who learns how bonuses work in one situation (such as for Armor Class) then knows how bonuses work in all situations. Then from that, penalties are very simple as they behave in the same manner. There are always exceptions to this (such as learning how most skills work doesn't apply well to the skill Speak Language), and exceptions should be kept to a minimum.
One of the larger exceptions in the combat system seems to be the grappling rules. Part of it being an exception is that it can seem to work differently depending on the attacker, and how you interpret the rules written for them.
[sblock](Not reformatted from the cache, so bear with it. Search on "carthain grapple" if interested.)
Provided below are new and hopefully improved rules for grappling in the d20 system. The d20 system was created originally with an eye towards the concept of mastery. Mastery is the idea that someone can obtain knowledge over a wide area of the game by learning one area. As an example, someone who learns how bonuses work in one situation (such as for Armor Class) then knows how bonuses work in all situations. Then from that, penalties are very simple as they behave in the same manner. There are always exceptions to this (such as learning how most skills work doesn't apply well to the skill Speak Language), and exceptions should be kept to a minimum.
One of the larger exceptions in the combat system seems to be the grappling rules. Part of it being an exception is that it can seem to work differently depending on the attacker, and how you interpret the rules written for them.
So this is an attempt to put a level of mastery into the grapple rules, as well as to be clear on how they work.
Grapple Checks Repeatedly in a grapple, you need to make opposed grapple checks against an opponent. A grapple check is a melee attack roll with a weapon (unarmed strikes are considered weapons), with a grapple modifier added on to it, and a special size modifier instead of the normal size modifier. The special size modifier for a grapple check is as follows: Colossal +16, Gargantuan +12, Huge +8, Large +4, Medium +0, Small -4, Tiny -8, Diminutive -12, Fine -16. (Use this number in place of the normal size modifier you use when making an attack roll.) A character with the Improved Grapple feat gains a +4 bonus to this check, as per the feat. Anyone may voluntarily fail a grapple check. Anyone can attempt to grapple with only one limb, or weapon they are holding by taking a -20 on all grapple checks. Doing this lets them retain their Dex to AC, and can threaten enemies around them that they can reach with other limbs/weapons.
Starting a Grapple To start a grapple, you need to grab and hold your target. Starting a grapple requires a successful attack roll with a weapon that you can grapple with. You can grapple with an unarmed strike, natural weapons, or any manufactured weapon that states you can grapple with it in the description of the weapon. If you get multiple attacks, you can attempt to start a grapple multiple times (each time with a different attack). Step 1: Attack of Opportunity. You provoke an attack of opportunity from the target you are trying to grapple (even if you have Improved Unarmed Strike, or are attacking with a weapon that does not normally provoke an attack of opportunity). Monsters with the Improved Grab ability do not provoke attacks of opportunity when they attempt to grapple, nor do characters with the Improved Grapple feat. If the attack of opportunity deals damage, the grapple attempt fails. If the attack of opportunity misses or fails to deal damage, proceed to Step 2. Step 2: Grab. You make a melee touch attack with the weapon you are attempting to grapple with to grab the target. If you fail to hit the target, the grapple attempt fails. If you succeed, proceed to Step 3. Step 3: Hold. Make an opposed grapple check as a free action.
If you succeed, you and your target are now grappling, and you deal damage to the target as if you had hit them with your weapon (the one you are grappling with).
If you lose, you fail to start the grapple. You automatically lose an attempt to hold if the target is two or more size categories larger than you are.
In case of a tie, the combatant with the higher grapple check modifier wins. If this is a tie, roll again to break the tie. (A grapple check modifier is the result of a grapple check, without the value from the d20 roll). Step 4: Maintain Grapple. To maintain the grapple for later rounds, you must move into the target's space. (This movement is free and doesn't count as part of your movement in the round.)
Moving, as normal, provokes attacks of opportunity from threatening opponents, but not from your target.
If you can't move into your target's space, you can't maintain the grapple and must immediately let go of the target. To grapple again, you must begin at Step 1.
Grappling Consequences While you're grappling, your ability to attack others and defend yourself is limited. No Threatened Squares: You don't threaten any squares while grappling. No Dexterity Bonus: You lose your Dexterity bonus to AC (if you have one) against opponents you aren't grappling. (You can still use it against opponents you are grappling.) No Movement: You can't move normally while grappling. You may, however, make an opposed grapple check (see below) to move while grappling.
If You're Grappling When you are grappling (regardless of who started the grapple), you can make any attack at -4 penalty with the following restrictions:
Can only attack with a light weapon or natural weapon (unarmed strikes are light weapons).
Cannot benefit from extra attacks from feats (such as Two-Weapon Fighting, or Cleave, or Whirlwind attack, etc.). If you start your turn with the intention of taking such extra attacks and then start grappling, then any extra attacks are lost (although any penalties remain).
You do not provoke any attacks of opportunity from anyone you are grappling with. You do provoke attacks of opportunity as normal for opponents who are not involved in the grapple.
You can perform any other action you are able to do normally if you make a successful grapple check. Some actions are restricted while grappling, and some new actions are available (they all take place of an attack unless otherwise specified): Activate Magic Item: You can activate a magic item if the activation time is 1 standard action or less. You must have the item in hand, or on your person as appropriate for activating the itme. If the item is a spell completion item, it cannot have a Somatic component, and you need to have any material components in hand or in place as required by the spell. To activate a spell completion item, you must make a Concentration check (DC 20 + spell level) or lose the charge/item. Doing this requires whatever action is normally required to activate the magic item. Cast a Spell: You can attempt to cast a spell while grappling or even while pinned (see below), provided its casting time is no more than 1 standard action, it has no somatic component, and you have in hand any material components or focuses you might need. Any spell that requires precise and careful action (DM judgement call) is impossible to cast while grappling or being pinned. If the spell is one that you can cast while grappling, you must make a Concentration check (DC 20 + spell level) or lose the spell. You don't have to make a successful grapple check to cast the spell. Doing this requires whatever action is normally required to cast the spell. Damage Your Opponent: While grappling, you can deal damage to your opponent equivalent to attacking with your unarmed strike. Make an opposed grapple check in place of an attack. If you win you deal either lethal or nonlethal damage (your choice) as normal for your unarmed strike.
Escape from Grapple/Pin: You can escape a grapple by winning an opposed grapple check in place of making an attack. Alternatively, you can make an Escape Artist check in place of your grapple check if you so desire. When making an Escape Artist check, your opponents do not get any size modifiers to their grapple checks, and you take a -5 penaly for each attack/grapple attempt you have already made this round. If more than one opponent is grappling you, your grapple check (or Escape Artist check) has to beat each grapple check made by your opponents involved in the grapple (if you are pinned, you only need to beat the grapple check made by the one grappling you). If you are grappling and you succeed, you immediately move into any space adjacent to your opponent(s) - this does not count towards your movement for the turn. If you are pinned and you succeed, you are now just grappling with your opponent(s) Move: As a move action, you can move at half speed (bringing all others engaged in the grapple with you) by winning an opposed grapple check. This grapple check is always at your highest attack bonus.
Note: You get a +4 bonus on your grapple check to move if there is only one other creature involved in the grapple, and if that creature is pinned (see below). Pin An Opponent: You can hold your opponent immobile by winning an opposed grapple check (made in place of an attack). Once you have an opponent pinned, there are only a few actions that opponent can perform Break Another's Pin: If you are grappling an opponent who has another character pinned, you can make an opposed grapple check in place of an attack. If you win, you break the hold that the opponent has over the other character. The character is still grappling, but is no longer pinned. Use Opponent's Weapon: If your opponent is holding a light manufactured weapon, you can use it to attack him with it. Make an opposed grapple check (in place of an attack). If you win, make an attack roll with the weapon with a -4 penalty (doing this doesn't require another action).
You don't gain possession of the weapon by performing this action.
Optionally, at DM's discretion, you may use an opponent's natural weapon against him.
If You're Pinning an Opponent You can attempt to damage your opponent with an opposed grapple check, you can attempt to use your opponent's weapon against him, or you can attempt to move the grapple (all described above). At your option, you can prevent a pinned opponent from speaking. You can use a disarm action to remove or grab away a well secured object worn by a pinned opponent, but he gets a +4 bonus on his roll to resist your attempt (see Disarm). You may voluntarily release a pinned character as a free action; if you do so, you are no longer considered to be grappling that character (and vice versa). You can't draw or use a weapon (against the pinned character or any other character), escape another's grapple, retrieve a spell component, pin another character, or break another's pin while you are pinning an opponent.
If You're Pinned by an Opponent When an opponent has pinned you, you are held immobile (but not helpless) for 1 round. While you're pinned, you take a -4 penalty to your AC against opponents other than the one pinning you. At your opponent's option, you may also be unable to speak. On your turn, you can try to escape the pin (see "Escape from Grapple/Pin" above).
Joining a Grapple If your target is already grappling someone else, you can use an attack to start a grapple, as above, except that the target doesn't get an attack of opportunity against you, and your grab automatically succeeds. You still have to make a successful opposed grapple check to become part of the grapple.
If there are multiple opponents involved in the grapple, you pick one to make the opposed grapple check against.
Multiple Grapplers Several combatants can be in a single grapple. Up to four combatants can grapple a single opponent in a given round. Creatures that are one or more size categories smaller than you count for half, creatures that are one size category larger than you count double, and creatures two or more size categories larger count quadruple.
When you are grappling with multiple opponents, you choose one opponent to make an opposed check against. The exception is an attempt to escape from the grapple; to successfully escape, your grapple check must beat the check results of each opponent.
Thanks to all those who helped me work on this and find and fix any problem areas.[/sblock]
