Trees, very rarely shrubs, deciduous. Twigs flattened at nodes, otherwise terete. Winter buds large, often superposed. Lvs opposite, imparipinnate, rarely reduced to 1 leaflet; leaflets usually serrate. Fls in terminal or axillary panicles, ☿ or unisexual. Calyx 4-lobed, small or 0. Corolla usually 0, sometimes of (2)-4-(6) distinct petals. Stamens usually 2, inserted on receptacle; filaments often long and exserted. Stigmas 2, < stamens. Fr. a 1-seeded samara with mainly distal wing.
Fls unisexual or seldom perfect; cal minute, 4-lobed or irregularly erose, or none; cor in our spp. none; stamens mostly 2; ovary bilocular, with 2 ovules per locule; fr a 1-seeded samara, the wing chiefly or wholly distal; trees with deciduous, pinnately compound lvs, the fls in dense fascicles, short racemes, or panicles, from the axils of the previous season. 65, N. Hemisphere.