You probably know that, in knitting, there are many kinds of increases, and that they all look different from each other. The complicated part is that the seemingly self-explanatory instruction "inc 1" can refer to any one of those increases. Fortunately, most patterns will specify which increase the designer intends in the glossary. Unfortunatley, vintage patterns and European patterns often expect a level of expertise or improvisational abilities above that expected by most modern American designers, and so leave out such life saving information. Now, to the heart of the matter:
- If "inc1" is defined as yarn over (yo) or make 1 (m1), the increase is made between two stitches, where no stitch previously existed. The yarn over is used primarily in lace patterns, because it leaves a large hole. The make 1 creates a much smaller eyelet, and is used in pieces where it is important for the shaping to be unobtrusive.
- When "inc1" is defined as knitting into the front and back of the stitch (also known as a bar increase), the increase is made in an existing stitch. This kind of increase leaves no eyelet in the fabric, but it does create a bump, kind of like a purl.
The point? If you increase with a make 1 when the pattern intends you to knit in the front and back, your pattern will be off as though you have one stitch too many. If a yarn over is intended and you knit in the front and back of the stitch, your pattern will seem to have one stitch too few. The devil is in the details.