the Reef knot
The reef-knot is only useful in simple applications. Ashley says "it is a true Binder Knot, for which it is admirable, but under no circumstances should it be used as a bend." It is easy tied and will not jam, so it is always easy to untie. It is used to tie packages, and as a base for he shoe-bow. Sailors used it for binding rolled sails or better reefed sails. And that is where it got its english name from. Americans call it the square knot. Probably because it looks square, or because it was much used on square-rigged-ships, but that is a total guess of me.
Its relatives, the granny, the thief-knot and the what-knot all have their purposes, but not as a trustful knot.
A few variations of the knot on video:
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