The crystal habit of a mineral describes its visible external shape. It can apply to an individual crystal or an assembly of crystals. In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen). The names of crystal habits are derived from: