Have you ever been out on the water and needed a rope of a certain length of rope only to have 2 short lengths of rope in the boat. I know that I have been in this situation and have obviously needed to join the ropes to make the required distance. I am no rope knot expert, in fact I am known for tying fairly poor knots, but even I use this knot!
One of the best knots for this task is the Zeppelin Bend, which can be used to attach two lengths of rope with similar diameters. It is easy to tie and quick to learn - the following video shows the process of tying the Zeppelin Bend very well.