A swatch is a small knitted sample (usually 15 to 20 stitches square) worked in the same yarn, needles, and stitch pattern you plan to use for your project. Its purpose is to measure your gauge: how many stitches and rows fill 10 centimetres of fabric, and whether that matches what the pattern was designed for.
If it doesn't match, everything else in the pattern is built on the wrong foundation.
This sounds like theory until you've knitted a sweater that comes out three sizes too large. Gauge isn't a suggestion from the pattern designer. It's the unit of measurement the whole pattern is built around. When a pattern says "cast on 90 stitches for a 50cm chest", that calculation assumed a specific number of stitches per 10cm. If your swatch gives you more or fewer, the maths doesn't hold.
The reassuring part is that gauge is adjustable. If your tension is tighter than the pattern calls for, go up a needle size. If it's looser, go down. Either way, swatch again before you commit. It takes an evening, not a week, and it's the only way to know before you've worked 20cm of body that something needs to change.
A swatch doesn't slow you down. It's the test run that saves you from starting twice.
There's something else swatching tells you that often goes unmentioned: how the yarn actually behaves. Whether it has elasticity or hangs heavy, whether the stitch pattern reads clearly or the yarn's texture obscures it. You find all of this out before you've committed to a full skein's worth of work.
Wash your swatch the same way you'll wash the finished piece. Yarns can change significantly when wet. Some merinos bloom and soften; others tighten slightly. Pin it out, let it dry fully, then measure. A dry swatch measured before washing gives you a number that may not survive its first wash.
How to measure gauge accurately
Lay your swatch flat on a surface it can't slide on. Don't stretch it. Place a ruler horizontally across the centre of the swatch, not near the edges where tension tends to drift, and count the stitches across 10 centimetres. Do the same vertically for row gauge.
Some patterns depend on row gauge as much as stitch gauge, particularly anything with shaped armholes or set-in sleeves. If a pattern governs length by measurement rather than row count, stitch gauge matters most.
When does gauge matter less?
For items without a fitted dimension like a scarf or blanket, gauge affects the finished size but not the fit. Hats still need the right circumference. The closer a project sits to the body, the more precisely gauge needs to match. A sweater leaves the least room for variation; a shawl the most.
The swatch is also where you meet the yarn properly. Give it the time it deserves.