Suppose the main terminal MT2 is positive and main terminal MT1 is negative. If the triac is now fired into conduction by proper gate current, the triac will conduct current following the path (left half) shown in Fig. 21.3 (ii). In relation to Fig. 21.4, the SCR1 is ON and the SCR2 is OFF. Now suppose that MT2 is negative and MT1 is positive. With proper gate current, the triac will be fired into conduction. The current through the devices follows the path (right half) as shown in Fig. 21.3 (ii). In relation to Fig. 21.4, the SCR2 is ON and the SCR1 is OFF. Note that the triac will conduct current in the appropriate direction as long as the current through the device is greater than its holding current