Adjustable shelving system having friction increasing mating surfaces

An adjustable shelving system has a plurality of generally cylindrical posts each having a plurality of vertically-spaced annular grooves therein and a plurality of tapered sleeves fitted on the posts. The tapered sleeves include an annular rib engageable with the annular grooves on the posts and an exterior mating surface. At least one plastic shelf has a plurality of tapered step holes for receiving one of the tapered sleeves fitted onto the post. The step holes are defined by an interior mating surface, and one of the exterior and interior mating surfaces is formed with a series of spaced cylindrical ledges to increase the friction between the sleeves and the step holes.