Hazelnut is the nut of the hazel and therefore includes any of the nuts deriving from species of the genus Corylus, especially the nuts of the species Corylus avellana. It is also known as cobnut or filbert nut according to species. A cob is roughly spherical to oval, about 15–25 millimetres (0.59–0.98 in) long and 10–15 millimetres (0.39–0.59 in) in diameter, with an outer fibrous husk surrounding a smooth shell. A filbert is more elongated, being about twice as long as its diameter.