Blame it on inconsistent definitions of yarn weights and a bit of knitting history that some people assume that yarn ply has something to do with the weight of yarn you're working with. The truth is a lot more complicated than that.
Back in the good old days when most people were knitting with wool yarn, ply actually did mean something when it came to the weight of yarn. A ply was always a consistent size, so a 2-ply yarn was very thin, while an 8-ply yarn was much bigger.
There's no such thing as a 1-ply yarn, by the way; technically, that's called a singles yarn. The singles are plied to create yarns of different thicknesses, but there's no longer any consistency as to the yarn's weight based on the number of plies.
A 4-ply yarn can be bulky or medium weight, while a singles can be super thin or super bulky, or anything in between for that matter.
Because that is the case, the old definitions of yarn weight as being a particular ply are disappearing in favor of the Craft Yarn Council of America's Standard Yarn Weight System. This system makes it possible for people from anywhere in the world to understand how thick or thin a yarn is because the system is based on numbers.
The system uses a scale from 1 to 6, with 1 being the finest yarn (also known as super fine) and 6, super bulky, being the biggest.
Should you see knitting instructions that call for a particular ply of yarn, however, these rules generally apply, based on the standards in the UK:
- 2-ply yarn generally refers to super-fine yarn
- 4-ply yarn is light weight (also known as DK or double knitting yarn)
- Aran yarn is not usually specified by ply, but this means worsted or medium weight yarn