Thread ply

[Orion]

I have two spools of thread, both polyester, both from the same manufacturer, both Tex 75 weight.
One is a 2 ply thread, the other 3 ply.

What's the difference in terms of strength, durability, sewability or any other factors that could matter?

[Moondog55]

That is a very good question
I'm sorry I cannot help as I have no knowledge but possible the multiple ply is more flexible

[gayet]

3ply is likely to be stronger than 2ply as the twist on twist (each ply twisted then all ply twisted to form thread) gives greater strength - to a point. Can result in a smoother surface less likely to be damaged, less chance of loose fibres etc.

[simonm]

I agree with gayet - I would say the 3 ply will be stronger and more durable.

[Orion]

So 3-ply is stronger and more durable than 2-ply.

Why would anyone buy the 2-ply thread?

[Moondog55]

Was there a cost difference?

[simonm]

So 3-ply is stronger and more durable than 2-ply.

Why would anyone buy the 2-ply thread?

It may depend on the intended purpose. There is a lot of decorative sewing that is not dependent on strength and durability.

[Orion]

The best prices at retail websites I could find were:

2-ply: $3.19 / 100m (only available in 100m spools)
3-ply: $3.29 / 400m (available in 400m and larger spools)

According to the manufacturer:
2-ply: Suitable for heavily stressed seams and rugged repair work.
3-ply: Very high tensile strength and wear resistance for durable seams.

[gayet]

As the threads are the same weight, TEX 75, each ply of the 2ply will be thicker than each of the 3ply. However, the 3ply will give a smoother overall surface and a better run through the needle and fabric. When using heavy fabrics, such as denim, the slightly rougher surface of the 2ply will be less of concern - there are bigger gaps between the warp and weft threads of the fabric.

[Orion]

Gayet, that makes sense. But it still doesn't explain why someone would prefer the 2-ply, at least I don't think it does.

Maybe it's easier to just tell you which threads they are:

Gütermann Extra Strong M 782 (2-ply)
Gütermann Tera 40 (3-ply)


Edit (10 March):
Out of curiosity I tested the breaking strengths.
Both the Extra Strong and Tera 40 broke at roughly 40N (4kgf).
The Tera 40 measured on average 6% stronger (between 2 and 9% out of three tests).
So, yes, a little bit stronger.