A flap on the edge of a book cover designed to be
inserted into the edge of the other cover. Its
purpose is to keep the covers of the book closed.
The extension is usually on the lower cover, and
tucks into a slit in the upper. The tuck was a
common feature of the bindings of mid-19th century
Bibles and prayer books, although it was rarely
used for the larger sizes. See also: CLASPS ; WALLET EDGE . (274 )