Blood Knot

This knot is used to join to lines of similar size together, also used frequently by fly fishermen tying there own leaders

Separate the lines in the middle

Place the ends thought the hole going in opposite directions. Grasp both center tag ends with your teeth and pull the lines to tighten. 

You can also tie this knot by making the turns around one line and placing the wrapped line in the top crease. Then winding the other line and placing it through the center hole going the opposite direction as the first wrapped line. Grasp the center lines with your teeth and pull the lines tight.