What ply means
Fiber is spun into a strand with twist; that strand is a single. Twist two or more singles together in the opposite direction and the result is a plied yarn — two-ply, three-ply, and so on. The plying twist balances the spinning twist, which is why a plied yarn hangs straight from the ball and a single, if it was spun firmly, curls back on itself.
In Britain and Australia "ply" also names a thickness — 4 ply, 8 ply — with no connection to the construction; the aran vs worsted guide sorts that out. Here it means the construction only.
How a single-ply knits
A single is soft and lofty for its weight, because nothing compresses the fibers. It blooms after washing into a slightly fuzzy, even surface that blurs stitch definition and hides uneven tension, and it feels warmer than a plied yarn of the same weight because it traps more air. Single-ply wool felts easily — the fibers are loosely held and ready to lock.
The costs: a single pills more, because the fibers on the surface are not anchored by a plying twist, and it is weaker, so it is not a sock yarn and not the yarn for a bag strap. A firmly spun single can bias in stockinette, the fabric leaning to one side; a lightly spun one, like most of ours, does not. Cables and lace show less crisply.
How a plied yarn knits
Plying makes the strand rounder and stronger. Stitches stand up individually, so cables, lace and textured patterns read clearly, and the fabric wears longer and pills less — a three-ply more than a two-ply, which is why sock yarns are plied and the best ones have three or four plies. Plied yarn has less halo and a little less warmth per gram, and it shows an uneven tension more honestly.
Which to choose
A soft, warm, plain sweater worn gently; a hat or cowl against the face; anything to be felted: a single. Socks, a cabled sweater, a child's garment, a bag, anything machine washed or worn hard: plied. For lace, a plied yarn blocks into cleaner lines; for colorwork, a single's bloom softens the join between colors, which some knitters prefer.
Singles we stock
Malabrigo Worsted and Malabrigo Rasta; Brown Sheep Lamb's Pride Worsted and Bulky (wool and mohair) and Top of the Lamb; Noro Kureyon, Silk Garden and Madara; Álafoss Lopi; Amano Yana XL. All but the Noro are classic felting yarns, and the felting guide is written around them.
Plied yarns we stock, at the same weights
Malabrigo Rios and Arroyo; Brown Sheep Nature Spun Worsted; Plymouth Worsted Merino Superwash; Schoppel Edition 3 (three plies) and Zauberball Crazy (two); the sock yarns on the sock shelf. The clearest comparison in the shop is Malabrigo Worsted against Malabrigo Rios — same wool, same weight, same colors, one a single and one plied — and the Rios vs Worsted guide is the worked example.