Abstract: A new lap joint is provided for fibreboard box construction having maximum strength and minimum cost, as compared with stapled, glued or taped joints long used in the manufacture of fibreboard boxes. The new lap joint construction involves thread-sewing lapped portions of a typical fibreboard cut flat, that is, a panel end portion at one end of the flat and a tab projecting from the other end of the flat are overlapped and sewn to form the joint. This new lap joint construction eliminates problems and avoids defects which long have existed in the art.