In the textile arts, plying (from the French verb plier, "to fold", from the Latin verb plico, from the ancient Greek verb πλέκω.) is a process of twisting one or more strings (called strands) of yarn together to create a stronger yarn. Strands are twisted together in the direction opposite that in which they were spun. Plied yarns will not unravel, break, or degrade as easily as unplied yarns. When enough twist is added to the plies to counter the initial twist of each strand, the resulting yarn is "balanced", having no tendency to twist upon itself.

The number of strands used to make the yarn is usually the same as the number of plies it has. Two-ply yarn means two strands were used, three-ply yarn means three strands were used, etc. Embroidery floss is generally a six-ply yarn, for example. There are some exceptions to this, most notably in chain plying.