It used to be that every knitting pattern, somewhere in the opening instructions, said something along the lines of "take time to save time" and "knit a swatch". I haven't seen it in the patterns I have been buying, but usually somewhere at the beginning the designer tells you what gauge they got so that you can attempt to match it.
For some things, it really doesn't matter! A shawl is a shawl, be it bigger or smaller, so you could (hypothetically) knit a shawl that was designed for lace weight yarn with size 2 needles in a worsted or bulky weight yarn and size 10 needles. Bigger? Certainly. Use more yardage? Definitely. Still a shawl? Absolutely.
But even for shawls, swatching is important. You might be using exactly the same yarn and needle size as the designer suggests in the pattern. But that doesn't mean that with your knitting style you will get the same "fabric" when you are done. And if yours is tighter or looser, will you like it?
There is another reason to swatch, when you are on the design side of things. It is possible to write up a chart, in excel or any other charting software, that completely, absolutely and positively adds up, but can't be knit!. Even when every decrease has it's opposite increase, and you think you have accounted for how stitches move in the fabric because of those increases and decreases, sometimes you just have to try knitting it to see how it really works out. (For instance, I can chart a beautiful cable in excel, let's say it is a 48 stitch cable, I can write instructions saying to slip 24 stitches to a cable needle and hold them to the front of the work, knit 24 from your left hand needle and then the 24 from the cable needle, but you can't do it! No matter how springy that merino is you have in your hands it will not stretch that far!)
Usually when I am designing a shawl I knit a pretty big swatch. Depending on pattern repeats (if there are any) I could be knitting a swatch that is up to half the size of my finished piece, just to make sure that all the elements line up the way I want them to. Sometimes it is lots of little swatches, just a few rows of one pattern detail then the next. I am pretty good at visualizing what the excel chart will look like knitted, but sometimes I want the security of knowing how it will look all knit up.
I didn't knit a swatch for the latest shawl creation before I started it.
Notice I said before I started it!
Now usually when you knit a swatch it is with the yarn and needles you intend to use for the finished project. But sometimes, when it is just checking that things that look good in excel look good in fabric, you can get away with cheating the way I am.
That grey shawl I started on Sunday, now has a baby sister. She is yellow and knit on much smaller needles (I was dying to try out the mini's I got last week) and I suppose you could say she is one section ahead of the grey one, although she is many rows smaller because I cut out the pattern repeats at the beginning. She is still a top down, center spine, triangle though. Because it was the spine I wanted to check.
Usually the spine is either just there, or incorporated into the body of the shawl pattern stitches. In this case, the spine is the pattern and the rest of the shawl is just filler.
The title up there, it is meant more for me than for you. I wouldn't be "leapfrogging" my knitting if I had bothered to knit the swatch in the first place!
Right now, the real shawl looks like any lace in progress. (In other words like knitting that the cat has chewed on and dragged around the house a few times!) The swatch.......well I think she is rather pretty!