A very useful knot for seizing the cut end of a line temporarily to stop it fraying or prevent the lay unwinding, or used in the bight of a line to enable the lay to be unwound back to a predetermined point prior to (say) forming a splice.
I find that the handiest way of remembering how to tie it is to form a clove hitch using two loops and to take the working end over the edge of the adjacent loop and tuck it back up through.
When the knot is drawn up taut it produces an overhand knot on top of a clove hitch, the two knots effectively locking each other. When tightening the knot ensure that the two parts of the whipping twine which pass behind the rope are lying snug against each other as to do otherwise can weaken the knot. This knot is usually so strong and tight that it can only be removed by cutting through the overhand knot.