The Albright Knot is a good knot for joining a main line to a leader or two lines of different diameter. It's not as good or strong as an FG, but a great alternative and a go-to when you are at sea and need a solid joining knot in a hurry.
Albright Knot:
1. Make a loop with the thick line and pass the thin line through and wind around.
2. Continue winding 8 or 9 times and poke the end out the opposite side.
3. Pull both together evenly and slowly.
Video: How to tie an Albright knot