Two Half Hitches
Two half hitches are used to make an adjustable loop. 
Two half hitches - simply named because the knot consists of a pair of half hitches.
Two half hitches are used frequently to create a simple adjustable loop.  An example would be tie a guy line around the top of a pole when pitching a tent or canopy.  Note that when subjected to a strain, the loop formed with two half hitches will tighten itself.
How to Tie

Stephen C. Dibler, Last update: 12/19/2002