The jury mast knot (or masthead knot) is traditionally presented as to be used for jury rigging a temporary mast on a sailboat or ship after the original one has been lost; some authors claim a use for derrick poles—but there is no good evidence for actual use.

Usually small blocks of wood are affixed to, or a groove cut in, the new mast to prevent the knot from sliding downwards.

Although these knots are tied in the bight, for the purposes of description the left side will be considered the standing part.

If all the crossings and overlays are reversed, or the right side is taken as the standing part, a mirror image of the knot will result.

This variation might grip the mast best (per Ashley) in the absence of other means to prevent the knot from sliding downwards.