Overview
Spoke nipples are typically T-shaped in cross section, with an internal thread running part of the way through the hole that runs along the spoke nipple's principal axis. A spoke nipple rests in a hole drilled radially through the wheel's rim, and the nipple is threaded onto the external thread of a spoke. The spoke itself is fixed, at its other end, through a hole in the hub. The spoke and nipple are functionally equivalent to a bolt and a nut. However, unlike a typical nut-and-bolt pair, a spoke and nipple do not join two parts (the rim and the hub) so much as bridge them, under tension.
The area of the nipple around which the spoke wrench, or key, is fitted in use is square in cross-section. Spoke keys may be 3- or 4-sided; the latter type has a small cutout to enable it to be fitted over a spoke, after which it may be slid along to engage all four flats of the nipple. It is less likely to round off the nipple than the 3-sided type, but is slower to use.
Read more about this topic: Spoke Wrench