Abstract: An interlocking paperboard carton blank (10) is disclosed. The carton blank has at least one flap (19) projecting from a side dge of a first base panel (14), the flap having a leading portion (20) which is folded about a scored waist section score line (23) back onto the remainder of the flap to double the thickness of the flap. The leading portion of the flap includes at least one, and preferably two laterally projecting portions (21). The flap is inserted folded end first into at least one correspondingly positioned opening (27) defined in a second base panel (15) of the carton blank.