Yarn ply and yarn thickness are not same! Technically the process of twisting the strands together is called 'to ply yarn'. A single strand of yarn is called a 'single' but most people will call it 'single ply' instead. 2 ply yarn is created from twisting two singles, 3 ply from 3 singles and so forth. Plied yarn is typically stronger than single strand yarn. But as mentioned earlier, there can be a lot of variation in strand size. A one ply yarn can be thicker than a 3 ply yarn, for instance. Thus, using ‘ply’ as a standard is problematic. Yarn thickness is measured using something called WPI, which stands for Wraps Per Inch. The idea is you wrap your yarn around a gap measuring 1 inch, and count how many strands you can fit in. Even to look at, it doesn't look like a yarn that should measure 16wpi, and be a Sport Weight.
A yarn's weight has nothing to do with what it weighs on a scale. What yarn weight is describing is the yarn's thickness—varying from yarns that are almost thread-like to others that are super bulky. Back 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 two-ply yarn was very thin, while an eight-ply yarn was much bigger. There's no such thing as a one-ply yarn. 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 four-ply yarn can be bulky or medium weight, while singles can be super thin or super bulky, or anything in between for that matter.
