A 'jam cleat' is a device which fixes a sheet or line in the desired position by 'jamming' or wedging the line into a narrow constriction.

The basic concept is to draw the line into a wedge-shaped groove fitted with gripping ridges or teeth, which help to draw the line downward into the narrower part of the wedge and keep it there. The more tension is put on the line, the deeper it jams into the wedge and the tighter the line gets fixed.

A jam cleat makes it easy to hold tension on a line with minimal strength and effort. Jam cleats are ideal sheet fixtures for mainsail controls on small boats where the sheet is often hand-held for extended periods.

The jam cleat allows the line to be adjusted toward the sailor without un-cleating.

A sailor can also simply pull the line upwards and out of the jam cleat's gripping teeth to instantly release the line.

"jam cleat" is a noun.