Shelf capacity increases as the unit descends - 11 lbs at the top, then 22 lbs, then 44 lbs on each of the bottom three - which matches how a leaning trapezoidal frame actually carries load and tells you where the heavy books belong. The shelves widen the same way, stepping outward toward the floor. Particle board and steel make up the 25 x 17.5 x 71 in frame at 31.5 lbs. It leans against a wall rather than standing free, so it needs a flat vertical surface behind it.