This document discusses the anatomy and operation of a triac, which is a type of thyristor used to control AC power. A triac is able to conduct current in either direction when its gate receives a trigger signal, allowing it to control both halves of the AC cycle. It can operate in any of its four quadrants depending on the polarity of the voltage across it and the direction of current flowing through it. The gate trigger must be applied during a small conduction window for the triac to change states.