From a simple picture, Spanish bayonet, Yucca aloifolia, is indistinguishable from the common giant yucca. The narrow leaves are about two feet long, and flare upward and outward from terminal buds. Plump conical trusses of waxy white flowers with purplish highlights stand vertically just above the foliage. The main difference is that the leaves are more rigid than they appear to be, with nastily sharp terminal spines.
The second most obvious difference becomes apparent as the trunks mature. These trunks get only about four inches wide, which is not quite stout enough to support their own weight. Some might get taller than ten feet, but eventually fall over. Terminal buds of fallen trunks curve upward, and try to grow vertically again. If not pruned away, old foliage browns and lays back against the trunks, like the beards of fan palms.