the.com/reef knot

Two loops that trust each other completely, until you pull sideways and they don't.

means A simple, symmetrical knot used to join two ends of rope of similar thickness, made of two overhand knots tied in opposition.

from The name comes from sailing: this knot was used to 'reef' a sail — tying up part of it to reduce its area in high wind. Sailors needed a knot they could tie and untie quickly and reliably even when wet and under strain, which is exactly what this knot offers — and exactly why it fails if you pull it the wrong way, collapsing into a useless slip rather than holding fast. It's also called a square knot, a name that points to its flat, symmetrical shape rather than its original use.

real name
square knot in most of the world
origin
sailors used it to reef, or shorten, sails
fatal flaw
slips apart under sideways or uneven load
not for
climbing or joining two different rope sizes
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