All members of this family have five-petaled flowers in which the superior ovary ripens to form a "pod", technically called a legume, whose two sides split apart, releasing the seeds which are attached to one or both seams. The legumous plants are classified into three subfamilies, sometimes raised to the rank of family in the order Fabales, on the basis of flower morphology (specifically, petal shape):
Faboideae (Fabaceae), previously called Papilionoideae: One petal is large and has a crease in it, the two adjacent petals are on the sides, and the two bottom petals are joined together at the bottom, forming a boat-like structure.
Caesalpinioideae (Caesalpiniaceae): The flowers are zygomorphic , but are very variable, e.g. closely resembling Faboideae flowers in Cercis, while symmetrical with five equal petals in Bauhinia.
Mimosoideae (Mimosaceae): The petals are small, and are frequently globose or spicate and the stamens are the most showy part of the flower.
A significant characteristic of legumes is that they host bacteria in their roots, within structures called root nodules. These bacteria known as rhizobia have the ability to take nitrogen gas (N2) out of the air and convert it to a form of nitrogen that is usable to the host plant ( NO3- or NH3 ). This process is called nitrogen fixation. The legume, acting as a host; and rhizobia, acting as a provider of usable nitrate, form a symbiotic relationship.