The surgeons knot is a very reliable knot for joining line and leader lines together. It's most effective when joining two lines that are the same, or no more than three times line tests apart. For example, 4lb line to 12lb, 8lb test to 20lb etc.
If you're fly fishing for bass this is a good knot for joining tippets together when building a tapered leader.
Don't get confused by the following instructions or knot tying illustration. The full length of the leader is not shown so remember that you must pass the end of the line as well as the full length of the leader through the loop on each wrap.
It's really a simple knot to tie.