Understanding yarn: weight, fibre and what the ball band is telling you

The first confusion is the word. Yarn weight means how thick the strand is, not how heavy the ball is. A ball of 4-ply and a ball of chunky can both weigh 100g; one contains far more metres of a much finer strand.

There is a standardised system of weight categories with numbers and names, published by a standards body for the craft, and most commercial yarn carries the symbol for it on the band. It is worth learning because it is the one part of the labelling that is consistent between manufacturers.

Ply properly means the number of strands twisted together to make the yarn. A 4-ply yarn has four. That is a fact about construction, not about thickness — four very fine strands make a thin yarn and four thick ones make a thick one.

In Britain the word drifted into being a thickness name as well, so "4-ply" is commonly used to mean a specific fine weight regardless of how many strands are actually in it. Both usages are in circulation and you have to read which one is meant from context.

Where ply genuinely matters is stitch definition. A yarn with more plies, tightly twisted, is round and firm and shows crisp stitches — which is what you want for textured crochet or cables. A loosely spun single-ply is soft and drapes and blurs the stitch pattern, which is lovely for a shawl and disappointing for anything relying on texture. It also splits under a hook, which is a common and unexplained frustration for beginners.

• Wool: warm, elastic, forgiving of tension — It stretches and recovers, which hides uneven tension and lets a garment be blocked into shape. It is the easiest fibre to learn on for exactly that reason. Some people find it itchy, and untreated wool felts if washed warm.
• Superwash wool: machine washable, and it drapes differently — Treated so it will not felt. It also loses some of the elasticity and tends to grow when wet, so a superwash garment can stretch out of shape in a way an untreated one does not.
• Cotton: crisp, heavy, no give at all — Excellent stitch definition, good for anything that should hold a shape, and the cause of a great deal of hand strain because it has no elasticity to absorb tension. Heavier than it looks in a large piece.
• Acrylic: cheap, hard-wearing, and not the joke it is treated as — It is the sensible choice for anything that will be washed constantly or dragged around — blankets, toys, children’s things. It does not breathe and it can go shiny and hard with heat, and a very cheap one feels like it. It is a legitimate material choice rather than a compromise.
• Blends: usually the practical answer — Wool with a little nylon for socks, cotton with acrylic for washability, wool with silk for drape. Most commercial yarn is a blend, and the band gives the percentages.

• Metres per ball is the number that matters for buying — Not grams. Two 100g balls of the same weight can differ substantially in length, and a pattern needs a length. If a pattern gives grams, work out the metres it means and buy on that.
• The recommended hook or needle is a suggestion — It is the manufacturer’s starting point for the gauge they quote. Your hands decide the rest.
• The dye lot decides whether a blanket has a stripe in it — The same shade dyed in different batches differs slightly, invisibly in the ball and obviously in a finished piece. Buy all of it at once, from the same lot, and buy one more ball than you think.
• The care symbols are the standardised ones — The washing, drying and ironing symbols on a yarn band are the same international system used on clothing labels, and they are worth knowing because they decide what you can make out of it. There is no point crocheting a blanket for a baby out of something that must be hand washed.
• To substitute: match gauge, then think about drape — Same weight category and the same stitches-per-10cm is the necessary condition. It is not sufficient — a cotton and a wool at identical gauge produce garments that hang completely differently. Swatch it.