In order to crochet a straight edge, and keep your number of stitches constant, you will need to construct the turning chain in one of two ways. The first is the most common and creates the straightest edge. The second is used to avoid the gap sometimes created when using the first method. The second is also often used when working in the round to keep intricate patterns even.
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Method One: Turning chain counts as first stitch -
Skip the first stitch of the previous row at the base of the turning chain. Crochet into next. When you reach the end of the row work into the top of the previous turning chain.
Method Two: Turning chain does not count as a stitch -
Work into the first stitch directly at the base of the turning chain. This counts as your first stitch. When you reach the end of the row do not work into top of previous turning chain.