Most thread for embroidery has a typical “Weight” of “40”. What’s this? Well, it comes from the old Japanese manufacturer Gunze’s numbered naming system for their thread (ex: #40), and the USA adopted an incorrect version of it, probably due to translation error, calling all threads by weight (ex: 40 wt.). Outside the US, the numbers are commonly sold using different systems.
Imagine if you will, being a thread maker. You need to buy the stuff to make your thread: cotton, silk, etc. You buy that stuff by weight, so it makes sense to sell your thread by weight too. And the idea that you have 1 gram of silk making up 100 meters of thread, gives your customers an insight into how thin, and how valuable, your thread is. Hence silk thread is often sold as 100wt.
Man-made material is measured similarly, but using the term Denier, which is the weight of 9,000 meters of thread. If 9,000 meters weighs 120 grams, that’s 120 Denier thread.
It would make sense if there was a common numbering system, and there is, called Tex (1,000m/g), but it isn’t as popular in use.
In embroidery, the traditional thread is rayon, and the 40 weight common thread is made by taking 2 thinner threads of 120 denier thickness, and winding them together. That makes the thread thick enough to embroider with a 75/11 needle and a .4mm density. When the makers of polyester began making embroidery thread, they approximated the rayon by using three pieces of 50 wt. thread wound together. Often you’ll see this as 50/3 on a spool. (Realize that 50/3 is 50% more than 50/2!)
But none of the systems actually address thread thickness directly. We can’t tell you why, other than that the thread makers aren’t thread users, and the thread users use their fingers to feel it.
You will notice that on some threads the thread weight is part of the name. This is important, as the program assumes a normal thickness (A.K.A. #40) but will read the “wt.” off thread names and adjust things such as short-stitching in native objects. More on that can be found in the section on the Thread Utility.