There was a sidebar about something else on 135
Dexterity Modifier: If your Dexterity is high, you are adept at dodging blows. If your Dexterity is low, you are inept at it. That’s why you apply your Dexterity modifier to your AC.
Note that armor limits your Dexterity bonus, so if you’re wearing armor, you might not be able to apply your whole Dexterity bonus to your AC (see Table 7–6: Armor and Shields, page 123).
Sometimes you can’t use your Dexterity bonus (if you have one) If you can’t react to a blow, you can’t use your Dexterity bonus to AC. (If you don’t have a Dexterity bonus, nothing happens.) You lose your Dexterity bonus when, for example, an invisible opponent attacks you, you’re hanging on the face of a crumbling cliff high above a river of lava, or you’re caught flat-footed at the beginning of a combat.