Many people find this an almost instinctive way to form a loop that tightens under load — a slip knot. It’s clearly based on the Overhand Knot and is an easy way to attach a line to a post or bar. To undo the knot, pull the sliding part back using the short end.
Slipped Overhand Knot
1. Form a crossed loop in the line so that the left part lies over the right.

2. Pass a bight of line from the left-hand part up through the loop.

3. Push more line up through the loop to increase the size of the bight.

4. The bight now becomes a sliding loop and can be adjusted to the required size, while the Overhead Knot formed in the working end needs to be pulled tight to finish the knot.

Knots in Action: The completed Slipped Overhand Knot ready for the loop to be dropped over a post or other fixed object, before the knot is slid up.
Read more from The Knot Tying Bible
Reprinted with permission from The Knot Tying Bible: Climbing, Camping, Sailing, Fishing, Everyday by Colin Jarman and published by Firefly Books, 2013.