Complex sentence: In the round, garter stitch is produced by knitting and purling alternate rows; by contrast, in the flat, garter stitch is produced by knitting every stitch (or purling every stitch, though this is much less common).
Split into simpler sentences: In the round, garter stitch is produced by knitting and purling alternate rows. By contrast, in the flat, garter stitch is produced by knitting every stitch (or purling every stitch, though this is much less common).