There are many kinds of yarn, in different textures and colours, and they differ in price as much as in anything else. Higher quality yarns carry a higher price tag than synthetics.
Using the exact yarn a pattern calls for usually gives the best result, but there will be times you choose something different. When you do, there are two things to match.
Match the weight
Choose a yarn as close in weight to the recommended one as you can. This is the more important of the two.
Weight determines gauge, and gauge determines size. Substituting a chunky for a double knit will not produce a slightly different garment – it will produce one that is wrong in every dimension, and no amount of needle adjustment will bring it back.
Compare yardage rather than ball weight. Two 100g balls can contain very different lengths depending on the fibre, and the pattern needs a length of yarn, not a mass of it.
Match the fibre
Choosing a yarn similar in fibre content will give a result that closely matches the pattern. This is the part people skip, and it explains most of the cases where a garment matches the stated gauge and still looks wrong.
Fibre determines how the fabric behaves:
Wool is springy and elastic. It holds ribbing and cables, keeps its shape, and recovers when stretched.
Cotton is heavy with no memory. It drapes well and it grows with wear.
Alpaca and silk are fluid and lustrous, with even less recovery than cotton.
Acrylic is hard-wearing, washable, and sits somewhere in the middle.
A cabled sweater designed in wool and knitted in cotton will have flat, sagging cables at exactly the right gauge, because cotton cannot hold the texture.
Price and quality
A higher price generally buys better fibre, better spinning and better dyeing – softer, more even, more colourfast, longer wearing.
But it is not worth spending on everything. Dishcloths, first projects and anything you are making to learn a technique are all better in inexpensive cotton or basic acrylic. Save the good yarn for things worn against the skin and for garments that take many hours, where unpleasant yarn wastes all of that work.
Always swatch a substitution
Work a square in the pattern stitch, wash and block it as you would the finished piece, and measure it then.
If your stitch count is higher than the pattern’s, go up a needle size; if lower, go down. And look at the fabric as well as the numbers – if it feels stiff or floppy compared to what the garment needs, the yarn is wrong however well the gauge matches.
