The core z20 rules do not cover all situations. Instead, as the GM Toolbox: Things to Remember states, GMs are expected to apply a ruling (usually a ±2 or ±4 modifer is sufficient) to those situations they feel don't quite fit the norm. The following document is a collection of such rules accumulated through play, as well as a few various other minor rules tweaks and options.
This document is not meant to be canoical, but simply an example of the sort of house rule document you may want to compile over time as a GM in order to remain consistent in your rulings. (Still, it's OGL, so take what you like.)
- BAB: Add all clvls from all classes with the same BAB progression before determining BAB. For example a 1-Clr/1-Rog would have a +1 BAB, rather than +0.
- Fighter: Weapon Specialization: In addition to gaining a +1 to at each even clvl, you may also move a single +1 previously selected from one weapon type to a different (even new) weapon type. The new weapon type should have been used previously/recently in play.
- Saves: A natural 1 on a save is always at least a failure; if it would have failed anyway, then it means a critical failure and something bad might happen: such as an extra effect or damage to a random item if the attack deals damage, etc. A natural 20 always succeeds; if it would have succeed anyway, it means a critical success: such as taking no damage/effect on an effect that allows a save for 1/2 dmg or partial effect, etc.
- Lucky PCs: If a single blow drops you from +HPs to 0 Strength, you are left at 1 Strength instead.
- You may choose to have a critical hit give you a free maneuver (as makes sense) instead of dealing double damage.
Special Combat Actions
- Block: -2 vs thrown weapons, -4 vs fired projectile
- Combat Trick: Each repeated trick in the same combat will likely suffer a cumulative -2 penalty, especially if vs. the same foe.
- Disarm: -4 if unarmed, but you end up holding the object if successful. +4 defense if holding the object with both hands. -4 to disarm with a ranged weapon
- +2 to a grapple attack if using two empty hands, or +4 if leaping onto your foe (though this leaves you flat-footed).
- If an attacker outside the grapple attacks one of the grapplers and misses by 4 or less, she hits the other grappler instead (as if that other grappler had been providing cover). (But see below.)
- You cannot grapple an opponent who is more than one size larger than you. You may use the same mechanic to cling to the opponent, the only real effect of which is that you add your weight to its encumberance.
- As an grappling attacker, you can use a variation of the block manuever: give up your all attacks this turn in order to attempt to prevent each of your victim's actions during the round with a successful grapple check.
- Move: -2 to ranged attacks vs running targets.
- Trip: Can be used to pull rides from mounts (though riders may resist with a riding skill check instead). Some creatures cannot be tripped (oozes, flying, etc), including most that are more than one size category larger than you. Some weapons can be used to trip, with the advantage that you can drop them to avoid being tripped in return on a bad failure.
- Either use the Aim action or take a -4 to attacks into melee with a ranged weapon or into a grapple with a melee weapon. If you don't, and if you miss by 4 or less, you strike the other combatant instead.
- Scraping the bottom of the barrel: Should a spellcaster ever exhaust his SP pool, he becomes fatigued and can cast no spells (even if they would cost him 0 SP). However, if a spellcaster ever lacks enough SP to cast a spell after including all his remaining (at least 1) SP, he may substitute HP for the remainding SP cost; however, this leaves him exhausted instead of fatigued. (Each player may choose whether or not their PC follows this variant--good for "innate"-style caster.)
- Upon gaining access to a new slvl, a character may swap a single signature spell for a different one.