What is Main Line?

Simply put, main line is the segment of line on your spool making up the majority of it. If you are casting, main line is the majority of what is flying through your guides. If you are trolling or vertical fishing that doesn't require casting, it is just the bulk of the line you have out. If you have a simple setup of just a single type of line on your reel, then that is your main line.

Why even have this special distinction of what is considered main line? There is no best type of fishing line, there are always trade-offs for what choice of line you make. As such, more complex line setups involving leaders, top shots, backing, etc have emerged. Main line is important because it will typically have the most impact on casting distance. It will also have the most influence on how your line reacts in the wind or water.

Fishing Fundamentals