More details about the The Blood Knot
Uses: The Blood Knot is a favorite knot for fly fisherman. It is primarily used to join two lines of similar size, e.g., when joining sections of leader or tippet, and is one of the best knots for this purpose. The strength of the knot depends on making at least five, and up to seven, turns on each side of the center
Tying it: There are several methods of tying it. The animation above shows each half being created separately, which provides a good picture of the structure.
Alternative: An alternative method is to just overlap the two ends and twist them together for about ten to fourteen turns. Then go to the center of the twists and create a hole. Pass the two ends the opposite way throught the hole.
Whichever method is used, the knot should be symmetrical about the middle with the twists continuing in the same direction either side of the center.
Advantages: The Blood knot is a simple, easily learned and very effective way of joining two similar sized lines.