Good yarn shops display to their customers samples of their goods : little square swatches (often 10 X10 cm) of the yarn they are selling, usually in stockinette, often fully enclosed by a garter stitch border. This is lovely ro see and touch, the start of many woolly love affairs, but I disgress. The key info is that swatches knitted for display in shops are ill-suited to measure gauge or determine if a yarn is suited for your project.
Stockinette only
Garter stitch and stockinette don’t have the same tension. Aka you don’t want to mix them in your gauge swatch. That garter stitch ? It’s a cofounding variable. It’s like trying to have a conversation with a friend despite someone else near you having a loud conversation on the phone. Inconvenient. Let your yarn speak to you in one-on-one conversation. For that very same reason, don’t include some special neat edge treatment. Knit every single stitch on the knit side, purl every single stitch on the wrong side. Measure your gauge (NImble needles has a beautiful tutorial here). That gauge is called the raw gauge. Then wash your swatch as you will do of your knit and measure again. That second measure is what yarn brand companies and knitwear designers are refering to when they talk about gauge. And that swatch test only refers to a stockinette gauge. If not, it is always specified.
Vogue knitting’s suggestion to “add two garter stitches at each edge to make it easier to measure” (p. 161, 2018 : Vogue Knitting. the ultimate knitting book) needs to be dismissed. I must say it is surprising it has withheld through its second edition, which is the version I own. . The American knitting guild association totally disapproves Vogue on that issue, so does Carol J. Sucolski (Yarn Substitution made easy, 2019, p.86). In fact anyone who bothers testing these different methods against each other will reach the same conclusion. .
Large
A 10 or 5 cm2 gauge is almost never enough, especially because it won’t allow much accuracy. The best place to measure your gauge is right in thethe middle of your swatch. so a swatch that is the size of your gauge is not going to be very helpful…. I usually cast on a swatch that has at least twice as many stitches as what the knitwear designer is suggesting for 10 cm. As June Hemmons Hiatt stresses in Principles of Knitting : any error in measuring your swatch will be multiplied by the full circumference and length of the garment! She recommends a large swatch as “a larger swatch means that if an error is made, it will represent a smaller percentage of the total measurement”.(p. 458) . In other words : some major fitting issue can be traced back to a gauge that is too small to provide any accuracy. Sucolski thus encourages us to see swatching as risk management and recommends a 6 by 6 inches swatch at the minimum.
As you will knit it further on : flat if you will knit flat, in-the-round if in the round. (See advice No.
It doesn’t need to be a square
When swatching for gauge, your are measure your stitch gauge (the number of stitches you need to reach 10 cm) as well as your row gauge (how many rows you need to have a knit 10 cm high). When unsure about the stitch gauge, I will cast on an approximative number of stitches to reach say, 20 cm long plus a few extra stitches for good measure. Then I knit a few cm. If I have a nudge the needle size is the one I will retain, I just cast off the few extra stitches and move on. If especially interested in discovering how drapey the fabric gets, I might cast off quite a few stitches and go on and on for a fairly long time. Also, even if I have knitted a square swatch, I keep it for further reuse. When I want to decide how to pick up the stitches, which cast off to use, which kind of decreases, etc, I go in all directions!
I’m not suggesting you should follow my lazy ways. Just keep in mind that you get decide which shape your swatch has. Sometimes you might want to be extra careful with your stitch gauge, sometimes you may need a lot of length to see how a yarn behaves, if it drapes well, if it has a tendency to grow, etc. Especially for plant fibre swatches and any alpaca blend, I think knitting a swatch taller than wide makes a lot of sense.
Unsolicited Knitting Advice 