Bench term
The Third Hand
Updated
The third hand is this site's name for whatever keeps a part in place after you let go: the tension of a thread, the friction between strands, or the memory of a crease.
In a sewn book
In a book sewn in sections, the third hand is the tension of the thread that links each section to the next. A 1982 bookbinding dictionary by Roberts and Etherington lists loose sewing among the causes of two failures: a shaken back, with gaps opening between the sections, and a start, where one section juts out at the fore edge. Bookbinding methods draws both and sets them beside the other ways a binding can grip its leaves.
Limits
The term names the holder; it does not replace the explanation. A page that uses it still says what the tension, friction or crease is doing and where it gives out, as when loose sewing lets the sections of a book work apart. See Which layer was checked for how structural statements are sourced.