A sheet feeder having a stack hopper for holding a stack of sheets, and feed belts for advancing the bottom-most sheet from the stack, is constructed to move at least one of the feed belts laterally toward and away from another of the feed belts while the belts are being driven. Preferably, side guides extend substantially to the end of the feeder, and extend both above and below the upper surface of belts to which they are adjacent. The belts extend between and around shafts of uniform diameter through the reach of lateral movement of the belts. The shafts are journaled in bearings mounted in such a way that they can be lifted out to change belts quickly and easily.