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Wiktionary
granny knot

n. A knot similar to the reef knot but crossed the opposite way.

WordNet
granny knot

n. a reef knot crossed the wrong way and therefore insecure [syn: granny]

Wikipedia
Granny knot

The granny knot is a binding knot, used to secure a rope or line around an object. It is considered inferior to the reef knot (square knot), which it superficially resembles. Neither of these knots should be used as a bend knot for attaching two ropes together.

Granny knot (mathematics)

In knot theory, the granny knot is a composite knot obtained by taking the connected sum of two identical trefoil knots. It is closely related to the square knot, which can also be described as a connected sum of two trefoils. Because the trefoil knot is the simplest nontrivial knot, the granny knot and the square knot are the simplest of all composite knots.

The granny knot is the mathematical version of the common granny knot.