This free pattern originally published in:
Spool Cotton #58, The Learn How Book 
Cast on the desired number of stitches and * knit one row, purl one row. Repeat from *.
Cast on the desired number of stitches and * knit one row, purl one row. Repeat from *.
Every row is knitted and two rows form a ridge.
When plain knitting is repeated back and forth any number of times, the work is alike on both sides and known as the Garter Stitch.