I'm not sure if this is common knowledge, but I've never seen it talked about. I think the majority of fly-fishermen use the perfection loop knot, especially in leader construction (please learn the knot from any internet search). This is a knot that works best when the loop is made small, but this can be tricky to do. The knot is correctly done when the tag forms a right angle with the main line.
I was building leaders today and realized folks may not know how to make a small loop. The above illustration shows the method. I use my thumb nail as a stop. If you make this stop and carefully pull the tag end before the knot is fully cinched down, the loop will reduce in size. When you have the size correct, cinch down the knot the rest of the way. Warning - if you pull too hard the loop may disappear altogether and you will have to start over.
Hope this is useful to you.