Round Turn & Two Half Hitches
This is widely used whenever a line has to be tied to a bollard, pile, rail, post or ring. It is popular for tying up a dinghy, securing mooring lines, fenders, awning lines, etc. The round turn is really two turns, developing enough friction in this way to resist a heavy pull and the two half hitches simply back up the round turn by keeping the tension on. The knot does not jam and is easily untied.