How to tell if a mystery yarn is thicker or thinner than another yarn

Ever wanted to use two yarns together in a warp, but wondered whether their thicknesses are compatible?

One way to judge the relative thickness of two yarns is to loop the end of one around the end of the other and fold both back so that you’ve got two strands of each hooked together, like so:

Then twist them all so that the two strands are plied into a single strand instead. 

Now that their grist is doubled, you may be able to visually inspect and see that one side is noticeably thicker than the other. If one is, then it’s probably thicker than the other, or at least will behave as such.

As well, you may be able to see that the twist travels into one yarn more than the other. In spinning, twist will move to the thinnest part of a yarn, and the same thing happening in your twisted yarns could be an indication that the more twisted yarn is thinner than the other. Be careful of this, though, since that can also be affected by how much twist was in the yarns to begin with, which direction each yarn is plied in, or other factors. If you’re reasonably sure that the two yarns have the same construction and ply twist, this might yield some clues.

If you can’t tell that one side is thicker than the other and the twist doesn’t move into one yarn more than the other, then chances are they two are close enough in weight to use them interchangeably in a project.

If you want to be even more sure, you can repeat this experiment with four or more strands of each yarn rather than just two.