The Round Turn and Two Half Hitches is a very quick and easy knot to use when taking a mooring line to a cleat or bollard. Just throw a full turn around the cleat and make a couple of half-hitches with the working end (or a bight) of the mooring line and the boat is secure.
The knot won't jam under load and is easy to release.
The Round Turn and Two Half Hitches can also be used to attach a line to any item of hardware, but there are other less bulky knots more suitable for this purpose.
For illustration purposes we're using a rail (well, to be truthful it's a plastic broom handle) rather than a mooring cleat.
Lay the line over the rail.
and take a full turn around it.
Take the working end around the standing part, making a half-hitch.
Do the same again, making a second half-hitch. Here we've gone from right to left around the standing part with the second half-hitch, but it's perfectly acceptable to go from left to right.
Hold the standing part and pull of the working end and you have the round turn and two half-hitches.
You can now relax in the knowledge that your boat is now securely tied to a plastic broom handle!