1-unit construction. No separate parts, preassembled so there is no contact under load except through the rolling balls. Center opening allows for center post and ease of centering the unit. Attaches easily to second surface even though hidden after it is fastened to first surface. Only 5/16" thick adds a minimum of height to item. (Can be recessed into surfaces, reducing space to 1/8".) Triangle's large ball race designed for maximum stability. Bearing is interlocked. No rivet to work loose or bind.