How many terminal leads does an SCR have?

A SCR or THYRISTOR has three leads, an anode and a cathode and the gate. It is a gate controlled diode which turns on once the gate voltage is reached and remains on until the current flowing through it is shut off. (either by another device or by an ac voltage across it passing though zero.) Large SCR's sometime have an extra lead but this is connected to the anode or cathode.