At the top of a knitting pattern, there's a little paragraph that a lot of us find puzzling or ignore. This paragraph is either of little practical use, or vitally important.
The information in question is the tension requirements. It'll say something like '22st and 30 rows to 4" using 4mm needles'. To further complicate matters, some of what's in that quote is of very dubious value indeed.
The first thing that knitters need to be aware of is what tension is. It's not about being a good knitter or a bad one. It's more like being tall or short. Just as we come in a whole range of heights, we all knit slightly tighter or looser than other people. If we're not following patterns, then any difference between our tension and someone else's is irrelevant.
But when we start to make things designed by other people that need to come out a particular size, the size of our stitches matter. That's what the tension information is: it's the designer telling you how big his or her stitches were, so you can get yours the same size.
So how do you check what size yours are? This is swatching, and it's a simple process. Cast on enough stitches to make a piece about six inches across, and work six or eight rows of garter stitch. Change to stocking stitch (or whatever stitch pattern the tension is given in) and work a square, keeping four or five stitches on each side in garter stitch. When you've made a square, work six or eight rows of garter stitch at the top and cast off (or not - see below).
Place your swatch on a flat surface and use a ruler to measure 4 inches across the fabric. Place a pin at each end of the measurement and count the number of stitches between the pins.
So why make a swatch larger than the 22 stitches and 30 rows? Knitting stitches get distorted at the edge of the row, so going wider and longer than those numbers gives you a more accurate measurement. And since stocking stitch curls at the edges the garter stitch border means that you're not trying to measure round corners.
If your garment is going to get bigger or smaller when you wash and dry it, the time to find out is right at the beginning. So the next thing to do is to wash and dry your swatch exactly as you will the finished garment. Then remeasure, with the ruler and the pins, and that's your tension.
If you've got the measurements the pattern calls for, then you can cast on in some confidence that you're going to end up with the size you want. But if you haven't, then the process needs to be repeated, with a different size of needle.
If you have fewer
stitches and rows than called for, then your stitches are bigger than the designer's and you need to make the smaller, so swatch with smaller needles. If you have more
stitches than the designer, try again with bigger needles to make yours bigger. After a while, you'll notice if you typically knit looser than most patterns call for or tighter, and you can start swatching on smaller or larger needles to save some time.
The reason for the large, three-decker swatch in the picture up above is simple: the knitter that made it is looser than most and has learned that a few attempts may be needed. So instead of dealing with a succession of swatches, it's quicker to make a square with one size, work a few rows of garter stitch, change needle and make another, and repeat, all in the same piece. All the measuring, washing, drying and remeasuring is then done at the same time.
For the record, this swatch, knitted with Knitpro interchangeable needles with Debbie Bliss Cashmerino DK, gave 23 stitches and 32 rows with 3.5mm needles, 22 stitches and 31 rows with 3.75mm and 21 stitches and 30 rows with 4.00mm. Those differences are enough to make a woman's jumper two inches too small, two inches too large or just right.
Why, you ask, make two sets of measurements? The second is your definitive one, but the first is what comes off the needles as you knit. As you make your garment, you can check that you're still getting the same straight-off-the-needles numbers as you work, and change needles if needed. Sometimes you relax as you get used to the project and loosen up, or sometimes stress in other parts of your life can tighten up your stitches.
But swatching tells you other things as well as the size of your stitches. This is why it's a first date with your yarn. Do you like working with it, enough to spend weeks with it? Does it bleed in the wash? Does it split? Does it make you sneeze? These are all things it's good to know at the outset.
On the other hand, sometimes tension doesn't matter at all. If your finished article doesn't need to come out a particular size, then there's no reason to be concerned about tension at all. How can a scarf not fit?
In this, swatching is exactly like measurements in cooking. A lot of the time, we don't need to measure our ingredients (when was the last time you weighed the eggs or measured the milk when making scrambled eggs?) - but measurements really matter if you're baking. That's what swatching is: it's setting the right oven temperature, weighing out your butter and using a quarter teaspoon of baking powder rather than a teaspoonful.