Page 9 - D. Raleigh "Knots and ropes for climbers"

Overhand Knot.

Similarly, you can tie an Overhand in each end of your rappel rope to help prevent the tails from
pulling through your rappel device. I've used the Overhand as a stopper knot once or twice in a rappel
rope, but only when I was in a hurry or not thinking right. In clearer moments you'll find that the
Double Overhand and special Stopper Knots work better, offering bulkier stoppers that are less likely
to untie.

Common Use
Stopper knot in the end of a belay rope

Secondary Use
Stopper knot in the end of a rappel rope

Single Overhands may slip
through a rappel device.

Double Overhand

The Double Overhand is a more useful variation. This knot takes only one more turn to tie than the
single but provides a bulkier stopper that is also less likely to untie itself. Outside the world of
climbing, the Double Overhand is tied in the ends of whips to prevent unraveling and add spice to the
taste of the lash.