This article describes a technique for interdigital web space deepening, which is applicable to incomplete syndactyly proximal to the proximal interphalangeal joint and to dorsal web space adduction contracture resulting from thermal injury. It consists of a dorsal rectangular flap and two palmar triangular flaps. The flaps are reversed to release dorsal adduction contracture. This procedure provides a broad commissure with a natural slope. A skin graft is avoided. A mathematical model shows that when the length-width ratio of the flaps is 1:1, the surface area of each flap is greater than or equal to the surface area of the defect it covers and the length of every flap edge is equal to or greater than its corresponding defect edge. If a web has a low apex angle and the skin is elastic, the length-width ratio may be as great as 1.5:1. The flaps will still cover the defects and small inequalities between certain flap and defect edges will be accommodated.