The reef knot is also known as a square knot; in fact,
the latter name is much more common (at least in my
experience, growing up in the USA and being an
Eagle Scout in the Boy Scouts of America).
Other useful knots for joining two ropes are the sheet
bend and the Carrick bend.
A rough ASCII art picture of one might look like this:
@@@@ ::::: @@@@@ ::::
@@@@@@ :::::::::@@@@@@@::::::
@@::::: :@@@@@ :::::@
::::@@@@@@@@@:::::::@@@
:::: @@@@@ ::::: @@@@
:::: @@@@
:::: @@@@@ ::::: @@@@
::::@@@@@@@@@:::::::@@@
@@::::: :@@@@@ :::::@
@@@@@@@@@@@@@@@ :::::::::@@@@@@@::::::::::::::::::
@@@@@@@@@@@@@ ::::: @@@@@ ::::::::::::::::
Notice that the ropes come out the same side they
came in; if you miss, you've created a granny knot.
Another problem can occur, even if you've routed the
ropes correctly: if you tighten only one rope, you
can end up with that rope going straight through, and
the other rope forming a lark's head hitch around
it. Usually, this is not what you want.