Abstract: A corner holder for use with a box of corrugated cardboard having a pair of slots in sidewalls thereof adjacent to a corner of the box, comprises a pair of inserts each including a tongue-shaped base plate and a U-shaped flange on the base plate, and a flexible connector web joined between the inserts. The base plate and the flange jointly define a channel or groove extending along opposite side edges and an arcuate bottom edge of the base plate. When the corner holder is installed on the box, the inserts are inserted in the slots, respectively, with edges of the sidewalls which define the slots being fitted in the grooves in the inserts. The corner of the box is thus clamped and reinforced by the corner holder. The box can readily be folded into a collapsed form simply by removing the inserts out of the slots.