To make yarn, fibers (wool, alpaca, cotton, bamboo etc) are spun into long strands (ply). These strands are then spun (twisted) together to form yarn. Single plys can be very fine (lace weight) to very thick (Islandic Lopi). Various yarns can be made of 2 - 20 plys. These yarns can be referred to as 2-ply, 4 ply, etc.

Using plys to describe yarn weights is still used in some parts of the world, but the meaning isn't always clear. The terms like "4-ply" or "8-ply" are used to describe the weight category of the yarn, not the actual number of strands.

In other parts of the world the terms lace, fingering, DK, worsted, or bulky, are used to describe the yarn's thickness