WvAC

WvAC stands for “weapon vs AC”, an important rule. WvAC comes into play any time the following two cases are true:

The attacker is not using a natural weapon (e.g. claws, bite).
The defender is wearing armor, or has a similar defense that is akin to armor, such as a beholder's plating.¹⁾

¹⁾

WvAC should NOT be applied to every monster, even ones with “tough skin”. WvAC should only be applied to armorless monsters when their defenses are said to be truly armor-like, such as the beholder's plating; MM 10: “The body is protected by a hard chitinous covering.”