I have tried to achieve straight edges by knitting into the back of 1st 2 and last 2 stitches of each row it seems to be a bit better but not completely straight.
-- Angelina, from the SweaterBabe.com Knitting Club
I haven't tried that trick... but I have taught students to make neater edges by increasing their tension just a bit as they do the first stitch on every row.
So, just pull your yarn a bit more (holding it with a tighter tension) when you are working that first knit or purl stitch. That can often help tighten up the edges and make them neater.
My suggestion does assume that you are holding your yarn consistently in your right hand (assuming you are a right-handed knitter!) to maintain even tension as you knit. If not, that may be something that you can practice to make things more even in general and therefore, your edges too!
Hope that helps!
Now - Stockinette Stitch edges are not all that pretty in general for anyone.
I rarely leave a Stockinette Stitch edge "raw" for a finished project. Usually, that edge gets seamed with another edge to form a side seam of a sweater or stitches are picked up along the edge to work a neckline or armhole edging. . .
The other "trick" that is used sometimes is called a "selvage" or "edge st". This means that the 1 st at either edge is worked only every other row on purpose to create a tighter looking edge.
To work a "knit" selvage, you can slip the first stitch on every row (as if you were going to knit it, i.e. knitwise), and then knit the last stitch on every row. This will create an edge that looks like a nice chain going up your work. Give it a try!
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