Taper

A taper is a device for inserting larger jewelry into a piercing.

Generally of stainless steel, a taper is a rod which increases in diameter from one side to the other -- obviously, the small end goes in first, and with the aid of lube, can be used to stretch the piercing up to the final larger size. The new jewelry is followed through after the taper, just like jewelry is followed through after a needle in a fresh piercing. Some tapers have threaded ends and can be physically attached to barbells to make the follow-through easier.