9 stitches + 18 rows = 4 inches [10 cm] in Garter stitch (knit every
row). When you match the gauge in a pattern, your project will be the size
specified in the pattern and the materials specified in the pattern will be
sufficient. The needle or hook size called for in the pattern is based on what
the designer used, but is not unusual for gauge to vary from person to person.
If it takes you fewer stitches and rows to make your swatch, try using a smaller
size hook or needles; if more stitches and rows, try a larger size hook or needles.
Making a Gauge Swatch