Dexterity Armor Class Bonus

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Armor, Armor Class, and Weapons - - - DMG

This bonus is in addition to that given by any other forms of protection.
The type of armor worn by the character with a dexterity armor class
bonus does not adversely affect this bonus, for it is assumed that his or
her physical conditioning and training compensate otherwise. (This is
particularly applicable with regard to magic armor which is assumed to
possess an enchantment which makes it both light and flexible.) The
penalty for wearing armor is already subsumed in the defensive bonuses
given for it, and if it were further to penalize the character by denying
dexterity armor class adjustments, it would be totally invalid.

Modifiers To Dexterity Armor Class Adjustment: Neither penalty nor
bonus due to dexterity (the Defensive Adjustment) is considered when
the character is subjected to the following attack forms:

    Attacks from the rear flank, rear, or strikes from behind (where the
        character is virtually unable to see the attack coming).

    Large missiles such as those hurled by a giant or some form of engine
        (where the trajectory and speed and size of the missile negate dexterity considerations).

    Magical attacks by spell, device, breath weapon, gaze, etc.
        (note that Defensive Adjustments do apply to saving throws for these attack forms).