The Numbers on a Thread Spool, Finally Explained

The first time most people pick up a spool of fly-tying thread, they expect the hard part to be the tying. Instead, it's the shopping.

By the J. Stockard Team

The first time most people pick up a spool of fly-tying thread, they expect the hard part to be the tying. Instead, it’s the shopping. A wall of little spools, each one stamped with a number (6/0, 8/0, 70, 140) and no obvious clue what any of it means or which one belongs on the hook in front of you. Worse, the numbers seem to argue with each other. It looks like a code that everyone else was handed at birth.

Yet, it mostly isn’t–there are basically two systems on the wall of spools, with a few specialty labels around the edges, and once a beginner can tell them apart, the whole exercise stops being so intimidating.

Here is what each number is actually saying.