A group of mules is called a span, a term borrowed directly from harness and draft-animal tradition. Specifically, “span” refers to a matched pair of mules yoked or harnessed side by side to pull a wagon, plow, or cart, working in unison with roughly equal strength and stride.

The word comes from the same root as “wingspan” or the “span” of a bridge, the idea of something stretching evenly from one point to another, which fits a well-matched team pulling together. Farmers and teamsters historically chose mules for this kind of work over horses because mules tend to be hardier, less prone to overeating or overheating, and more resistant to disease, making a reliable span of mules a valuable asset on any working farm or ranch.

While “span” specifically implies a pair, larger mule teams are sometimes called a pack or a string, especially when multiple pairs or a longer line of animals are involved.