[1] Synapomorphies such reduced staminate flowers, advanced wood anatomy features, and the presence of spermidines in pollen define the Coryloideae.

[2][3][4][5] A few authors have divided the genera into two families, the Betulaceae (Alnus, Betula) and Corylaceae (Carpinus, Ostrya, Corylus, Ostryopsis).

Corylus is placed as the sister group to the remaining Coryloideae because it shares plesiomorphic character states with the Betuloideae such as bisexual inflorescences, staminate flowers with a perianth, a haploid chromosome number of 14, and nonoperculate pollen apertures with thickened endexine.

[2] The monophyly of Corylus species is defined by several morphological synapomorphies, including the large animal-dispersed nuts, hypogeal seed germination, and filaments that are completely divided longitudinally.

[2][3] Several characters, including glabrous receptacle of staminate flowers, smooth nutlet surface, and secondary veins extending directly into leaf teeth, differentiate Ostryopsis from Carpinus and Ostrya.

[2] Carpinus is defined by several morphological characters including presence of pistillodes in the male florets, and leafy bracts subtending the flowers and fruits.

[2][5] Although both genera share similar inflorescences, pollen grains and vessels with simple perforations, they differ in leaf epidermal characters and their infructescence bracts, which are radially symmetrical and inflated bladder-like in Ostrya and are open and flat in Carpinus.