Shrubs and trees , 3--15 m; tree trunks usually 1, branching mostly deliquescent, trunks and branches terete. Bark grayish brown, thin, smooth, close, breaking into vertical strips and scales in age; prominent lenticels absent. Wood nearly white to light brown, moderately hard, heavy, texture fine. Branches, branchlets, and twigs nearly 2-ranked to diffuse; young twigs differentiated into long and short shoots. Winter buds sessile, broadly ovoid, apex acute; scales several, imbricate, smooth. Leaves on long and short shoots, 2-ranked. Leaf blade broadly ovate with 8 or fewer pairs of lateral veins, 4--12 × 3.5--12 cm, thin, bases often cordate, margins doubly serrate, apex occasionally nearly lobed; surfaces abaxially usually pubescent, sometimes glandular. Inflorescences: staminate catkins on short shoots lateral on branchlets, in numerous racemose clusters, formed previous growing season and exposed during winter, expanding well before leaves; pistillate catkins distal to staminate catkins, in small clusters of flowers and bracts, reduced, only styles protruding from buds containing them at anthesis, expanding at same time as staminate. Staminate flowers in catkins 3 per scale, congested; stamens 4, divided nearly to base to form 8 half-stamens; filaments very short, adnate with 2 bractlets to bract. Pistillate flowers 2 per bract. Infructescences compact clusters of several fruits, each subtended and surrounded by involucre of bracts, bracts 2, hairy [spiny], expanded, foliaceous, sometimes connate into short to elongate tube. Fruits relatively thin-walled nuts, nearly globose to ovoid, somewhat laterally compressed, longitudinally ribbed. x = 11. Corylus differs from other Betulaceae in various features, most notably in the infructescences, which consist of small clusters of well-developed nuts, each enclosed by a loose involucre of leaflike bracts. As in Ostrya , the staminate catkins are formed during the summer and are exposed through the winter prior to anthesis. In Corylus , however, pistillate catkins develop at the same time as the staminate, and they consist of only a few flowers, protected by the scales of special buds rather than being arranged in elongate pistillate catkins. The staminate flowers are unique in the family in that well-developed sepals are occasionally present, clearly defining the three individual flowers that make up each cymule. A longstanding disparity occurs in the literature regarding the diploid chromosome number found in Corylus species, with both 2 n = 22 and 2 n = 28 being cited. J. G. Packer (pers. comm.) believes that the 2 n = 28 for several species (R. H. Woodworth 1929c) was in error because of a misinterpretation of Woodworth's meiotic preparations, a number of which actually indicate eleven haploid chromosomes. Woodworth's count may be largely, if not entirely responsible for the persistence of this number in the literature.
The genus consists of three major subgroups, the first composed of shrubby plants having a short, open involucre of two bracts surrounding the fruits ( Corylus sect. Corylus ). Members of Corylus sect. Tuboavellana Spach are of similar habit but have the involucre modified into a tubular beak, and Corylus sect. Acanthochlamnys Spach is characterized by densely spiny bracts. Recent treatments have avoided applying sectional names. The genus as a whole should be considered for taxonomic revision.
Corylus is the source of hazelnuts and filberts. Commercial filberts ( C . colurna Linnaeus and C . maxima Miller) are cultivated in various parts of the world, particularly Turkey, Italy, Spain, China, and the United States. Wild hazelnuts ( C . americana and C . cornuta ) are smaller but similar in flavor to those of the cultivated species.