The double uni, or uni-to-uni, joins two lines by tying a uni knot with each line around the other and sliding them together. It is easy to learn, easy to tie on the water, and strong enough for the vast majority of connections, which makes it the everyday leader knot for a lot of anglers.
When and why to use it
Use it to join braid to a mono or fluorocarbon leader, or to splice two similar lines together. It is the reliable all-purpose choice when you do not need the ultra-thin, guide-friendly profile of an FG knot. The trade-off is bulk: the double uni is a bigger knot than the FG, so it clicks through the guides more, which matters most when you are casting the connection a long way.
How it works
Overlap the two lines, tie a uni knot with the first line’s tag around the other line (several wraps), then tie a matching uni with the second line around the first. Lubricate both, snug each knot down, then pull the standing lines to slide the two knots together into one compact connection. Use more wraps for thin braid.
Tip Double the braid before you tie the braid-side uni for extra grip and strength, and take a few more wraps on the braid side than the leader side. Trim the tags close so the knot slides through the guides more smoothly.