What is a 2-ply? A 2-ply is a yarn with two single yarns spun together. If the single was Z-spun, then you will S-ply, and if the single was S-spun, then you will Z-ply. Plying creates a balanced yarn.
2-ply yarns are stronger and more consistent than singles. Their surface has more texture than yarns with more plies. Plus, they have more potential for color and design effects than singles. They are a great choice for lace because 2-ply yarns flatten out when blocked and allow the fibers to bloom within the stitch. They are excellent yarns for weaving.
2-ply yarns lack the roundness that helps cables and other knit-purl stitch patterns pop. Also, maintaining the sweet-spot of ply balance can be tricky, for 2-ply or other multi-ply yarns. Overplied or underplied yarns can result in a biased knitted fabric.
In Amy Tyler’s “Ask a Spinning Teacher: Plying for Balance” from Spin Off Winter 2016, she says, “If your goal is a balanced yarn, the amount of twist you add when plying depends on the amount of twist you put into the singles. Once you have decided on the twist in the singles, the direction and amount of twist in the plied yarn is more or less decided for you. Basically, the more tightly twisted your singles are, the more ply twist you need to get balance.” It is a good idea to make a sample and check your yarn against the sample often.