As promised, the next instalment for beginners is here. Tension is one of the most important things in both knitting and crochet. However, it is a little complicated, and can easily be misunderstood. The first part of tension lies in how a crafter holds their yarn. The tighter you hold it, the tighter your stitches will be, and the less yarn you'll use. If your stitches are too tight, however, they will be extremely difficult or even impossible to work.
Using this information helps you figure out if the garment you're making will the the right size. While many things don't require gauge swatches, such as scarves, blankets, shawls and stoles, other things, especially sweaters, really need them. The pattern will include the required gauge/tension. It will be in the form of 10 cm/4 inches = x stitches over y rows, usually similar to the gauge on the ball band of the yarn suggested in the pattern.
The swatch is easy enough to work. You cast on the number of stitches the tension of the pattern suggests, and knit the required number of rows. You then cast off and measure the swatch. It's strongly suggested in most circles that you also wash it, to give a good indication of what the material will do once it's been put through what the finished garment will go through. If it measures 10 x 10 cm (4 inches square) then you are set. But what if it doesn't? If it is too big, go down a needle size. If it is too small, go up a needle size. You may have to try a few times to get the right size needle if your tension is not typical to what is on the ball band, or if you are using a different yarn to that suggested in the pattern. For example, a DK weight yarn called for in the pattern might be slightly thicker or thinner than one you decide to use instead, and so the tension will be slightly different.
A note on yarn substitution: if you are going to use a different yarn to make a project than one suggested, it is a good idea to use one of the same weight. Using a different weight yarn will change the math of the pattern, as you will need to cast on more or less stitches for a thinner or thicker weight of yarn. While you would need to do a tension swatch to know how many stitches you'd need to cast on and work, this is a little more advanced and shouldn't be tried until you are very comfortable with how patterns fit together.
I will be the first to admit that I don't always swatch. I know my tension usually matches that of the ball band, and I can alter things if I find my pattern going awry while I am knitting. This comes with practice and confidence. I still get caught out, however, and have had some catastrophes in my time. I tend to swatch for sweaters and hats for the most part, and always when I made things for other people. Tension swatches are a good idea for making sure things fit and look their best on the intended wearer, and should be used accordingly.