Stopper knot

Stopper knot
Names	Stopper knot, Backup knot, As numbered in picture : 1 – Fiador, 2 – Sailor's diamond (#693), 3 – Figure-eight loop, 4 – Diamond, 5 – knife lanyard, 6 – Chinese button, 7 – Chinese button doubled, 8 – True lover's, 9 – Ashley's, 10 – Celtic button, 11 – Celtic button on the bight (and thus doubled and with lanyard loop), 12 – Friendship, 13 – Figure-eight, 14 – Overhand
Typical use	Keeps the line from slipping out of things.

A stopper knot (or simply stopper) is a knot that creates a fixed thicker point on an otherwise-uniform thickness rope for the purpose of preventing the rope, at that point, from slipping through a narrow passage, such as a hole in a block. To pass a rope through a block, or hole, is to reeve it. To pull it out is to unreeve it. Stopper knots prevent the rope from unreeving on its own.

"Stopper" has three distinct meanings in the context of knotting and cordage. A decorative stopper knot may be referred to as a lanyard knot.

The single-strand stopper knot is...[one variety] of knob knots. Generally it is tied as a terminal knot in the end of a rope, where it forms a knob or bunch, the general purpose of which is to prevent unreeving. It is found in the ends of running rigging. It secures the end of a sewing thread; it provides a handhold or a foothold in bell ropes and footropes. It adds weight to the end of a heaving line, and it is often employed decoratively, but it should not be used to prevent unlaying and fraying except in small cord, twine, and the like, as a whipping is in every way preferable for large and valuable material.

— The Ashley Book of Knots^[1]

A monkey's tail "is a permanent or semipermanent stopper that is put in the bight as well as the end. It is also called single throat seizing, seized round turn, clinch, and pigtail...A small round turn is first taken, and a throat seizing, in length about a quarter of the round of the clinch, is put in. The monkey's tail is preferred for the purpose just described because it does less damage to rope than any knot. When the monkey's tail fetches against the rack the seizing takes the burden."^[2]