You mount it on the top of a door, vertical, pin pointing up, chain hanging
down. It has a chain hanging down with a ring. The upper end of the chain
is attached to a slider bolt. The upper end of the bolt is shaped like the
part of a door closure that runs along, then into the pin hole to hold the
door shut. Usually used on the second door of a set of two to hold the top
shut when there is not a big need to have it shut really tight or secure.
First door has passage lock or exterior door knob lock on it. Available in
large sizes, too.

What are they called?