Swatch. There I said it. Not a favorite topic among knitters for sure. I think that most knitting teachers out there (at least in the classes I have taken) do a really good job of stressing the importance of swatching before beginning a project in order to check your gauge. I have found though that books are bad about breezing over the subject. Of course you always see "Take time to check your gauge". In fact we see that statement so often that we tend to breeze over it. It's like that note pinned to your fridge that is no longer needed but it's been there so long that you don't really see it anymore. I don't want to bore you with the details of HOW to do your swatch and measure your swatch, but rather point out the importance of the swatch. I learned the hard way. I have made numerous sweaters that did not exactly fit the intended wearer. So much wasted yarn and wasted time. Let's say that you are making a sweater for a 36" bust. Now follow me here.... Your needed gauge is 22 sts per 4". That will mean that you will have a total of 198 sts across the bust line because 22 sts divided by 4" gives you 5.5 sts per inch. Your bust is 36" multiplied by 5.5 sts gives you 198 sts.
Now if your actual gauge is 20 sts per 4" you would have only 5 sts per inch. That's not much right? Only 1/2 stitch difference. But keep in mind that it's a 1/2 stitch difference for every inch and you have 36 inches. At your actual gauge you would have only 180 sts across the bust. That is an 18 stitch difference in the number of sts at the bust. So if you ignore your gauge and go ahead and cast on according to your pattern then your sweater will be 3 1/3" larger than you wanted.
Enough of the boring stuff. What do you do with all those swatches. I know some knitters who catalogue their swatches and label them with the needle size, yarn used and stitch pattern. I save my swatches however I do not catalogue them. I plan to use my swatches to Yarn Bomb a chair. Stay tuned for that adventure!