The first number is fineness
In the English cotton count system used for spun polyester, a higher number means a finer single yarn. 20S is coarse, 60S is fine. The number describes the single before it is twisted, not the finished thread.
The second number is ply
The number after the slash is how many singles are twisted together. 20S/2 is two singles of 20S. 20S/4 is four. More plies make a rounder, stronger, heavier thread at the same nominal fineness.
Why ply matters more than buyers expect
A three-ply thread is rounder in cross-section than a two-ply of the same weight. Roundness is what lets a thread pass a needle eye thousands of times without flattening and fraying. This is why 40S/3 exists alongside 40S/2.
Matching count to fabric
The rule of thumb is that the thread should be lighter than the fabric it joins. Heavy denim and canvas take 20S constructions. General garment assembly sits around 40S/2. Fine shirting and lingerie need 50S and 60S so the stitch line does not stand proud of the cloth.
S and Z twist
Twist direction has to match the machine. A thread twisted in the wrong direction untwists as it feeds and starts breaking. Both S and Z are available and the direction belongs in the purchase order, not in an email afterwards.
