A useful knot for tying two ropes (of similar size) together. I find that it jams up less than the square knot (a.k.a. reef knot) and is almost as easy to tie.

In loving ASCII art:

         ______  ______  ______  ______________
        |..... \/      \/ .... \/
        |____ . \  __   \. __ . \  ____________
             \ . \/  \   \/  \ . \/
             /\ . \  /\   \  /\ . \
            /  \ . \/ .\   \/  \ . \
           /   /\ ./ . /\  /   /\ . \
          |   /  \/ . /  \/   /  \ . |
          |   \  / . /\  /   /\  / . |
           \   \/ . /. \/   /  \/ . /
            \   \. /\ . \  /\   \. /
             \   \/  \ . \/  \   \/
  ___________/\   \__/\ . \__/\   \____
  .............\       \ ......\       |
  _____________/\______/\______/\______|


Notice that the long ends come in on opposite sides of the whole knot, and that the short ends exit opposite each other. Also, each rope crosses over itself once.