When the white surface is rubbed or scratched with a sharp implement, it changes from light to dark brown, producing visible lines and shading. Cap varies widely from 5 to 75 cm across; more or less fan-shaped, semicircular, or irregular; with a dull, unvarnished outer crust; often furrowed in "zones"; brownish to grayish brown. Pore surface is white, becoming dirty yellowish or dingy brownish to olive in age; bruising brown; with 4 to 6 tiny (nearly invisible to the naked eye) circular pores per mm; tubes in layers (a new layer is added each year), separated by brown tissue, with each layer 4 to 12 mm deep. Stem is usually absent; if present, lateral and stubby. Flesh is brown to cinnamon brown (rarely whitish); very tough.
It is a wood-decay fungus, using primarily dead heartwood, but also as a pathogen on live sapwood, particularly on older trees. It is a common cause of decay and death of beech and poplar, and less often of several other tree species, including alder, apple, elm, horse-chestnut, maple, oak, walnut, and willow.