Ply Number; Cables and Lace

Over on the Hilltop Cloud Ravelry group a discussion topic popped up about ply number, and what would be best for a cardigan with cables...
Of course common wisdom is that you should use 2-ply for lace, and 3-ply for cables. But a picture speaks a thousand words. These are all swatches I made up for my Spinning with a Purpose workshop, over time I'm sharing the swatches on via a series of blogposts, you can read the previous posts here. 
We'll start off with the lace. Here's a side by side shot. 3 ply is on the left, 2 ply on the right.

Same thickness of yarn, same needles, but the holes are definitely more visible on the 2ply swatch, and the decrease columns more visible on the 3ply swatch.

Close up of the 2-ply swatch.
​You can see at the point where the strands wrap over at the yarn over they nestle together. 2-ply yarn isn't actually round, it's more like an oval shape that spirals, that allows the strands to sit together nicely.

In contrast on the 3-ply swatch those twisted sections take up far more room, and that means the hole you create from the yarn over is far less visible. What is very apparent though is how much that centred double decrease pops proud of the surface, and that feature is what makes 3-ply yarns a star for cables and textured stitches. 
Texture is very hard to see in photos, trying to demonstrate just how good a 3-ply is at cables is somewhat tricky, I wish you could all feel these swatches for yourselves.

This is a standard 3x3 cable, together with a column of twisted stitches. In both cases the motif really pops off the surface of the knitting. 

In the 2-ply it's ok, partly because I like a firm tension for my knitting, but it lacks that 3-D pop.

A shot from side on hopefully demonstrates what I'm on about... 3ply on the left, 2 ply on the right. It's not that the 2-ply version is rubbish, just that it lacks the wow factor that adding the extra ply produces.

So there you have it, sometimes the collective wisdom is wrong, but in the case, most definitely not. 2 ply for lace, 3 ply for cables!