bolt
The folded edge at the head, tail, or fore edge of
a section of an unopened book. Depending on
location, it is known as a head bolt, tail bolt,
or fore edge bolt. Bolts are generally located at
the head and fore edge, but cannot be located at
all three edges in any one section. The folded
edge opposite the fore edge is not a "bolt," but a
spine fold, or "last fold." Bolts are often opened
quickly and with little care, resulting in ragged
edges that are difficult to handle when turning
leaves. A dull knife used carefully will result in
a clean cut, while a knife that is too sharp is
difficult to control and often cuts away from the
bolt and into one or more leaves.