There are several different ways to crochet into a chain, and all produce different results. My favorite for creating a nice, finished looking bottom edge is to work into the back bump.
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Working into the Back Bump Photo Tutorial
(scroll down for video tutorial)
Each chain is made up of three separate strands of yarn. From the front, you can see the two strands you are most familiar with. The top loop and bottom loop make up that “V” shape we all know and love.
If you flip the chain around to the back though, you will see a third, hidden loop. This is sometimes referred to as the third loop or back loop, or my favorite, the back bump.
Crocheting into this back bump leaves that V in the front completely undisturbed. So when you finish your row, that V is still showing on the bottom of your work.
Since most stitches have the same V shapes on the top, the bottom edge matches the top edge nicely. This means you can leave the bottom edge as-is and it will still look finished, without the need to work an edging.
Working into the Back Bump Video Tutorial: