What ply knitting yarn is this?

I have discovered that yarns in Australia are classified in a different system than we have here (in the centre of the universe!) in North America! :-)

When I think of "ply", it denotes how many strands of yarn there are. But you use that term to describe weight of yarn.

When substituting a yarn, you should go by what you want to do with it. If the pattern suggests gauge (for sizing purposes), find a yarn that will give you a gauge very similar. But also determine how it behaves when knit. Some yarns stretch, some will fluff up when washed, etc. You then have to adjust the size of needle or number of stitches or the yarn to get what you need at the end.

With experience, you will gather enough information about a variety of yarns that you have available, and the appropriate gauge for them, and you will be able to easily switch one yarn for another. But at your present stage, find a yarn you may like, make your gauge swatch, and see how it works. Make any adjustment in needle size to make it work. Or change to a different yarn to get better results. You can sometimes use 2 or more fine yarns together to get a heavier weight.

Most patterns are produced by a particular yarn company, so of course, they are going to recommend that you use THEIR yarn. You may do that, or sometimes you want to use what you already have in your stash, or what is available in your area. As long as you get what you want in the end, what you use is all up to YOU. Keep that in mind.

In UK terms, Baby Econo is a 10 ply, it's a worsted weight, 9 wraps per inch. Gauge to 10 centimeters is 20 stitches. Here's the Baby Econo page at ravelry.com, if you're a member:

http://www.ravelry.com/yarns/library/red-heart-ltd...

You wouldn't buy it if you could feel it, it's a dry, scratchy yarn that I wouldn't put next to a baby's skin.

Look at the tension given in the pattern.

You may use any yarn that gives the same tension. This info is usually given on the ball band.

Medium-4 if that helps you any