Ply

"Ply" is an imprecise description of yarn weight. You should always check the required tension gauge if you are substituting or choosing yarn for a particular pattern.

Beginner Crocheters & Knitters Please Note. Australia uses different yarn weight ply’s to American & English wools. Most of these differences are in the wool & free patterns around the world.

     An example of this is a USA Worsted or DK yarn is an 8 ply yet here in Australia a worsted yarn is classed as a 4ply. The same as with English crochet stitches in patterns they too are different to Australian. If you are a beginner please take the time to teach yourself regarding yarn weights & ply’s.
The World yarn council are slowly unifying this problem around the world but until they do please take the time to learn about Australian Yarn Weights & Patterns plus World-wide yarn weights & patterns, so you understand the pattern and yarns that you are using. Most European countries are now in line with Australian yarn weights & Ply's.
If you have doubt always knit a tension gauge square and adjust your needles or crochet hook. Doing this will save you a lot of time, money, & failures. You will also have more success with patterns fitting correctly with the yarn that you are using.
You will develop your own knitting style & Tension as you learn.