Have you ever noticed that your (row) gauge is different when you work in the round? It is for most knitters, even if just slightly, and that's mainly because our tension varies between knit and purl stitches. If you're, for instance, making a stockinette stitch sweater with the front and back knit flat and the sleeves knit in the round, it is very likely that your row gauge for the sleeves will be different from the front and back. And the same goes for swatching! So when you're swatching for a new project (and you're always swatching, right?), it's recommended, and I dare say important, to always swatch the way the project is worked, meaning to swatch flat if your project is knit flat, and to swatch in the round if your project is knit in the round.
Now swatching in the round might look a bit messy and intimidating, but I promise, it's a piece of cake once you've got the hang of it! Plus not having to rip back because you made a quick & dirty flat swatch and got a different gauge will be so worth it.
You'll need: a circular needle with a cable length of at least 40 cm, yarn (I'm using De Rerum Natura's Ulysse in the caramel colourway, scissors.
Cast on the number of stitches needed for your swatch. I used a Longtail Cast-On here, but any cast-on suited for knitting flat will work.
Step 1: Do not turn your work (like you normally would) and slide the stitches to the right ...
... until they are on the opposite end of your circular needle.
Step 2: Bring your yarn to the right end, leaving a long piece of yarn (I like to leave a loop that's slightly longer then the width of my swatch) ...
... and prepare for working the next row.
... to prevent them from loosening. This will give you a much neater edge.
Repeat steps 1-3 until you reached the desired length.
Bind off all stitches.
Give the loose strands a little tug, adjusting the tension on the first and last stitches.
And you're done! Now it's time to cut the loose strands so your swatch can lay flat and you can block it easily.
This is what your final swatch will lool like.