Vertical stripes (narrow)
To knit narrow vertical stripes (4 stitches wide or less) you need two balls of yarn. You carry the yarn you are not using across the wrong side of your work. The image shows vertical stripes done with stocking stitch.
To knit wider stripes, or multiple colours, you use individual bobbins of yarn for each vertical stripe, and a method called intarsia to twist the yarns together as you change from one colour to the next.
Abbreviations: k = knit, p = purl, (A) = in colour A, (B) = in colour B
To knit vertical stripes, 2 stitches wide, in two colours, in stocking stitch:
Cast on an even number of stitches.
Row 1: *k2(A), k2(B), repeat from * to end, carrying the yarn colour you are not using to the back of the row.
Row 2: *p2(A), p2(B), repeat from * to end, carrying the yarn colour you are not using to the front of the row.
Repeat rows 1 and 2 for pattern.
As you carry the yarn you are not using to the wrong side of your work, make sure you do not pull it too tight. If you pull it too tight your work will bunch up. It helps if you keep your stitches stretched apart from one another on the needle as you pass the yarn to the wrong side.