The simple Simon under bend is a knot belonging to the category bend. It was invented by Harry Asher. It is more secure than the similar Simple Simon over and more effective with quite large differences in thickness of the two ropes.[1]
The simple Simon under holds well even with different sized ropes, or slippery synthetic ropes.[2]
Tie as shown in the images. Note, that, as in the sheet bend, the two running ends should emerge on the same side of the knot. [3]