5.5 Block (Rule 15)

Blocking is the action of players close to the net to intercept the ball coming from the opponent by reaching higher than the top of the net (Rule 15.1.1). Only front-row players are permitted to complete a block; a back-row player or Libero who completes a block commits a fault (Rule 15.1.3).

Source: Rule 15.1.1, Rule 15.1.3, Rule 15.1.2, Rule 15.4, Rule 15.4.1, Rule 15.4.2, Rule 15.4.3

Send this to your friend — argument over.