Works on multiples of six stitches.
Knit three, purl three across.
Ribbing can be made in just about any width, including uneven numbers like a 5x2 ribbing, where you would knit five stitches and purl two stitches across the row. These bigger ribs are often used as a motif across a whole garment or knitted article rather than being used just as a hem or cuff like more traditional and smaller ribs are.
Making a blanket or sweater that is all ribbing can give the piece a dramatic and sophisticated look.