The joining of two lines, has been in the past, the hardest knot for fishermen to tie quickly, or the worst knot for strength, they can tie.  A knot used for joining two lines needs to be very small in diameter, so it can go through rod eyes, and be wound into reels, without hanging.  The EZ knot line to line knot, is simple to tie, very fast, and very strong. It is simply making four wraps around the tool, and running the end under the wraps, holding the knot on the tool with your index finger slide the other line under the wraps on the other side of the tool. Slide the knot off the tool and pull up around the second line.  Then just tie the same knot around the first line with the second one, pull up tight, then slide the two knots together, you can cut the tags right up against the knot, so you will have nothing that will catch in your rod eyes.  This knot can be used for different line sizes, and/or materials.

Due to the small size of the knot, this works great for leader, to tippet, when fly fishing, and for attaching shock leaders, or for backing on the reel.

From left to right
50# to 12#
12# to 12#
50# to 50#
12# to 100# braided