Girth Hitch is also known as a Cow Hitch or Lark's Head. This combined with a simple half-hitch to lock it in place is the most useful knot in the world imho.

Girth Hitch is also known as a Cow Hitch or Lark's Head. This combined with a simple half-hitch to lock it in place is the most useful knot in the world imho. Don't use a double fisherman or water knot unless you're okay with cutting the rope afterwards to untie it (if you've jammed it or gotten it wet). Tying a back-up knot into your trace-eight isn't necessary, but people have died from not having one (when they screwed up the eight), and it doesn't take long to do. Alpine butterfly is a very useful knot for tying into the middle of a line. You'll use it more in life than in climbing. A Hasty Harness or Swiss Seat is an easy way of tying someone into a rope or webbing if a harness breaks or is missing and you need to rappel or haul a person.