What is a swatch?
A swatch is a small piece of knitting that is used to measure your gauge — the number of stitches in a piece of knitting 10cm wide. You can also measure your row gauge — the number of rows you have in a 10cm length.
If your gauge matches the gauge given in the pattern, then your knitted garment should be of the expected size.
If you have more stitches per 10cm than the required gauge, and you were to just cast on the number of stitches given for your size, your garment will come out smaller than you want.
If you have fewer stitches per 10cm in your swatch then your garment will be larger than you want.
If your gauge isn't right, change needle size as necessary, and knit a new swatch.
Benefits of swatching
- Firstly of course, you will find out if your gauge matches that given in the pattern—especially important if you're using a substitute yarn
- ...and therefore find out if you are going to make a garment that looks and fits as you hope
- Get to know your yarn and see whether you like the fabric that you get from a particular yarn/needle combination
- Try out any new stitch patterns, so you make mistakes in the swatch rather than the real thing
- Decide whether you like the yarn and pattern before you commit to casting on a whole sweater's worth of stitches
- Wash the swatch and find out how the fabric changes and whether you are making something that is easy enough to keep clean
It's up to you, of course. If you're knitting a shawl or a blanket and the finished object ends up 5 or 10% bigger or smaller, you may not mind. Even so, there's a chance you'll use significantly more or less yarn than expected if you're working at a different gauge.
The more experienced the knitter, the more likely they are to swatch. They probably learnt the hard way that getting to know a yarn with a swatch is the first step towards a successful project.