The acorn, or oaknut, is the nut of the oaks and their close relatives. It usually contains a seedling surrounded by two cotyledons, enclosed in a tough shell known as the pericarp, and borne in a cup-shaped cupule. Acorns are 1–6 cm long and 0.8–4 cm on the fat side. Acorns take between 5 and 24 months to mature; see the list of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.