A method of sewing a book by hand in which two sections are sewn on a single length of thread from kettle stitch to kettle stitch. After the first (or first two or three) section is sewn ALL ALONG , two sections at a time are placed in the sewing frame, and the sewing alternates from section to section. There are several techniques of sewing two on, a common method being to alternate from one section to the next at each cord. Whatever the method used, two sections receive as many stitches as does one in the all along method, and when sewing on five cords, alternating sections received two and three stitches, respectively.
Sewing two on was quite common until the last quarter of the 19th century, at which time sewing machines began to be used in trade (edition) binding. Even the thinnest books were sewn two on, not so much to reduce sewing swell as to reduce costs. Sewing two on today is used in fine binding only to reduce swell. Also called "two sheets on" and "off and on." See also: THREE ON . (94 , 236 , 335 )