Stacking in d20

The issue of stacking in d20 came up at the weekend. This is controlled by the following rules:

1) Most bonuses, and several penalties, have names attached. So, you get an armor bonus, a shield bonus, a deflection bonus, and so on.

2) Bonuses with the same name don't stack, except for circumstance and dodge bonuses. Penalties always stack. (If two named bonuses apply, you always use the biggest.)

3) Unnamed bonuses stack with everything except themselves. (So, if you cast Roger's Spell I, which provides an unnamed bonus to attack rolls, it stacks with all your enhancement and circumstance bonuses, but you can't then cast it again to get double the bonus.)

And that's about it. This is why studded leather armour doesn't stack with bracers of armour - they both provide an armor bonus to AC.

This seems nice and simple in concept, but has a couple of hidden complexities:

1) Enhancement bonuses generally don't apply to a quality, but rather apply to an object that then applies to a quality. So, studded leather armour +1 has a +3 armor bonus normally, plus a +1 enhancement bonus. The enhancement bonus adds to the armor bonus, to make the total armor bonus +4 for that suit of magic armour. This allows a magic suit of armour to stack with a magical shield - the enhancement bonus for one applies to the armor bonus, the other applies to the shield bonus.

2) Those two exceptions - circumstance and dodge bonuses really screw everything up. Why they didn't just make them unnamed bonuses, I really don't know.

The reason that named bonuses don't stack is to prevent some really stupid situations, such as characters wearing multiple suits of armour for added protection. It also has some neat effects, in that some types of bonus are harder to get than others (for instance, only magic rings provide a deflection bonus, and they cost twice as much for the same benefit). However, it could, surely, be better explained.