Here in Australia, we are very used to the Ply system of yarn weight measurement, which has been in use for decades. The word ply originally referred to the number of strands used to create the thickness of yarn. So a 4ply used 4 strands, and an 8 ply would use 8 strands. However, more and more yarns are being produced using modern processes which can manufacture a single stranded very thick yarn, or a chain yarn of any thickness, so that while we still use ply to describe the thickness of a yarn, the term is not strictly accurate anymore. Also, many patterns are coming from other parts of the world using terms like lace weight, sport weight, and Worsted, which often double up. For example, a pattern which calls for a Worsted weight yarn, will do equally well with an Aran Weight, or 10ply of similar tension.
To help to deal with the confusion that all of these terms can cause, Craft Yarn Council have produced a new Standard Yarn Weight System. This system uses a simple icon to identify the yarn weight between 0 (Lace) to 7 (Jumbo/Roving).
Here you can see the icon for Yarn Weight 4, a medium weight yarn which we would call Worsted or Aran weight. It can also be called Afghan weight. We would expect this yarn to use 4.5mm to 5.5mm needles to produce a tension of 16 to 20 stitches over a 10cm test square.
At Knit & Yarn, we are educating our customers about this new system, as it is integrated internationally and used with our favourite brands. Our website is transitioning to use this system too.
Source: Craft Yarn Councils www.YarnStandards.com