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Midshipman's Hitch or Tautline Hitch

This hitch is really just an adjustable loop used for non-critical applications where you need to adjust the size of a loop to apply tension to guy lines, like on your tent.  After you tie it, grab the bulk of the knot and slide it where you'd like.  Once you release it, the loop will hold how you've sized it.  There are two common ways to tie this knot, and both serve the purpose.

The knot must be drawn up very snugly to work, and may not work at all on expecially stiff or slippery rope. Don't expect too much from it.  It's not a very secure loop.  If you tie down a load on the back of a truck with this, you're likely to find that your lines have gone slack after a few miles.  A Versatackle is a better choice there.

You'll find a few pictures of how to tie the Tautline Hitch here.