Renthal Threadless 31.8 mm 50 mm 100 deg Bicycle Stems
Bar clamp diameter is the diamater of the handlebar.
Stem length is measured from the center of one clamp to the center of the other clamp. This measurement is sometimes known as the reach or extension.
Stem angles are measured perpendicular to the clamp. For example, a 90 degree stem is straight. Sometimes, this may also be refered to as a 0 degree stem. If two measurements are listed, the stem can be flipped for angling up or down.