Haikomi-giri

Finger joints, known as haikomi-giri, are a fundamental joint technique used in traditional Japanese carpentry. To create a finger joint, the end of one wooden piece is precisely cut at an angle into a series of projecting "fingers," while the end of the adjoining piece is cut to create complementary "notches" that the fingers will interlock within. The joint gains much of its strength from the direction of the wood grain and fibers within the joint. Since each finger sits at an angle, the grain on the adjacent sides of a finger always flows in opposite directions when fitted into place. This prevents the joint from failing due to seasonal movement that causes wood to expand and contract across its grain. The opposing grain directions effectively cancel out any expansion or contraction forces, helping to ensure the joint will remain tight and secure over time. The meticulous cuts and snug interlocking fit achieved through finger joints allow wooden structural elements like beams, rafters, and columns to be joined without glue, nails or other fasteners, demonstrating the fine woodworking skills of traditional Japanese carpenters.