When two alternating quantities attain their respective zero or peak values simultaneously, the quantities are said to be in-phase quantities. When the quantities do not attain their respective zero or peak values simultaneously, then the quantities are said to be ou-of-phase quantities. The quantity which attains the respective zero or peak value first, is called ‘Leading Quantity’. The quantity which attains the respective zero or peak value later, is called ‘Lagging Quantity’. In above diagram, it is seen that for inductive circuit, the current is lagging behind the voltage or the voltage is said to be leading the current. Similarly, for capacitive circuit, the current is leading the voltage or the voltage is said to be lagging behind the current.