A rivet is a permanent mechanical fastener. Before being installed, a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite to the head is called the "tail". On installation, the rivet is placed in a punched or drilled hole, and the tail is "upset", or "bucked" (i.e., deformed), the head is supported against the direction of the tail when upsetting so it will stay on place, the tail expands to about 1.5 times the original shaft diameter, holding the rivet in place. In other words, the pounding or pulling creates a new "head" on the tail end by smashing the "tail" material flatter, resulting in a rivet that is roughly a dumbbell shape. To distinguish between the two ends of the rivet, the original head is called the "factory head" and the deformed end is called the "shop head" or buck-tail.