Most knitting pieces start out as rectangular or square in shape.
Most knitting pieces start out as rectangular or square in shape. Many beginners find that one or sometimes both edges are sloping at a tangent rather than forming a straight edge. Also the stitch count increases.The garter stitch sample starts out with ten stitches. The most common cause for an edge growing is improper placement of the working yarn as the row is started. I have knit only one stitch of the new row, but you can see that there are two stitches on the right needle. This happened because the yarn was in front of the needle as I started to knit. Basically a yarn over at the beginning of a row; note the large loop along the right selvedge which would be the yarn over for lace or button holes.
The first six rows on the bottom of this five stitch stockinette sample are correct with three stitches running up the center of the sample. The green line on the right shows that the knit rows are started properly as one stitch to the right of the green line runs for the length of the sample.The yellow line shows that the left selvedge is growing as stitches keep appearing to the left. Even beginners finish a row properly. It is the first stitch of the new row that can present problems. Thus we know that a purl row is the issue here; the working yarn for a purl row must be on top of the needle and the working yarn pulled down as described below.
There is a second cause for extra stitches on the edges. This picture shows the working yarn pulled straight up;
this gives the appearance of two stitches. It is easy to knit into both those legs and increase the stitch count. Pulling the edge of the knitting down will force that working yarn back where it belongs as is shown in the last picture.