The simple simon under bend is a knot

Knot

A knot is a method of fastening or securing linear material such as rope by tying or interweaving. It may consist of a length of one or several segments of rope, string, webbing, twine, strap, or even chain interwoven such that the line can bind to itself or to some other object—the "load"...

 belonging to the category bend. It was invented by Harry Asher. It is a more secure than the similar simple simon over. The difference is just which of the green ends in the image below is the standing end.

The simple simon under holds well even with different sized ropes, or slippery synthetic ropes.

Instructions