I came across this today and thought I'd share. I'm sure for most, it's general knowledge but just in case anyone might find it useful:
As for what ply a yarn is - that terminology is fast becoming obsolete with all these fancy acrylic yarns. The best way to know is by making a tension swatch, especially if you are trying to substitute these new yarns in older patterns.
A rough guide is as follows:
29-32 stitches over 10cm = 3ply, baby yarns, fingering weight and heavy crochet cotton
25-28st over 10cm = sport yarn, sock wool, 4ply and 5ply yarns
21-24st over 10cm = 8ply, medium weight
17 - 20st over 10cm = 10ply, aran, heavy worsted
13 -16st over 10cm = chunky, bulky, 12ply+
9-12st over 10cm = super chunky, super bulky
I hope this is useful to you. You can check the ballband tensions against this guide so you have a better idea.