A spur is an equestrian device attached so as to protrude beyond the heel of a rider's foot and is used to goad a horse into moving at a speed faster than it is currently travelling. Early spurs are of the prick type and have simply pointed ends often with a rounded section immediately after the prick probably in order to reduce the chance of actually puncturing the skin of a horse's flanks.
In the late 13th century the rowel type spur was developed. This is shaped like a wheel with numerous spikes projecting from the edge. The rounded shape again prevented the spikes from entering too deeply in a horse's flank.