There are four broad categories of meteorites—chondrites, achondrites, irons and stony irons. These are subdivided into meteorite groups, the basic unit of meteorite classification. Although no formal guideline is in place for the minimum number of meteorites needed to define a group, common practice is that there should be five or more members in a group. A defined meteorite group is thought to be derived from a single asteroid. However, some groups are genetically related and are derived from a common parent asteroid. Chondrites are primitive stony meteorites; rocks whose compositions are little changed since their formation in the solar nebula. There are fourteen defined groups of chondrites, and they make up the vast majority of meteorites falling to Earth in the current epoch. Achondrites are stony meteorites of two broad types. Some are primitive materials like chondrites, but most are the products of igneous differentiation. There are ten defined groups of achondrites, of which seven are differentiated types. Irons are also the products of asteroidal differentiation, having crystallized from metallic melts separated from chondritic precursors. There are thirteen defined groups of iron meteorites. Stony-irons are also differentiated materials, and both the rocky and metallic phases were formed by igneous processes on asteroids. There are two defined groups of stony-iron meteorites. In addition to those that fit into groups, there are many meteorites that are unique, or for which there are less than five examples. These ungrouped meteorites make up a substantial fraction of meteorites recovered to date.