This is based on my write-up entitled "kettlestitch" on Everything2. E2 does not permit illustrations; thus the ASCII art. I will tailor the text and add photos at a later time.
Bookbinding - A row of chain stitches that holds the signatures of a book together. Most rows of kettlestitches are sewn in grooves cut into the backs of the signatures.
The term comes from the German word ketteln, meaning to link or make a chain.
This description assumes you have already sewn three signatures, and are now facing your first kettlestich before you go on to the fourth.
CCCCC CCCCCCCCCCCCCCCC / n / \ / \| \ |\ | \ B|BB\BBBBBBBBBBBBBBBBBB | \$ | $ | $\ \ $ \ \ $ / ---- $ AAAAAAAAAAAAAAAAAAAAAAAA n = needle $ = knot