granny knot

This is what people often tie when they really meant to tie a square knot (which is also known as a reef knot). Unless you know what you're doing, avoid using this knot.

An ASCII art version:

         ______  ______  ______  ______
        |..... \/      \/ .... \/      |
        |____ . \  __   \. __ . \  ____|
             \ . \/  \   \/  \ . \/
             /\ . \__/\   \__/\ . \
            /  \ ......\       \ . \
           |   /\______/\______/\ . |
           |  |                  |. |
           |  |  ______  ______  |. |
           |   \/ .... \/      \/ . |
            \   \. __ . \  __   \. /
             \   \/  \ . \/  \   \/
  ___________/\   \__/\ . \__/\   \_____________
  .............\       \ ......\
  _____________/\______/\______/\_______________

Note that the loose ends of each rope come out on the other side of the loop as they came in; this is the sign of the granny knot.