There are two fairly easy methods for creating a ribbed edge with crochet.
The first method is explained
in this photo tutorial and works by making stitches in only the back loops of the previous row, at right angles to the main body of your project.
Sometimes this isn't practical for the item you are making or perhaps you want to have horizontal stripes across your ribbed edge. In cases like these, this second method works really well!

Detail of Method 2 Ribbed Edge
For this tutorial, I have made a Basic Beanie Hat - please
click here for instructions on how to crochet one - but decided to finish it off with a striped ribbed edge to match the stripes of the beanie.

Beanie with striped ribbing
Materials
Co-ordinating or contrasting yarn - amount needed will vary depending on the size of item you are ribbing
Appropriate hook for yarn
Tapestry needle
Stitches used
(In UK/Australian terms)
YO Yarn Over
Ch Chain stitch
Slst Slip stitch
Tr Treble
BLO Back loop only - stitch is worked in the loop furthest from you, not the whole top "V" of the row below
FPTr Front Post Treble - stitch is worked by inserting the hook round the post of the stitch from the front, not under the top "V"
BPTr Back Post Treble - as for FPTr but the hook is inserted from the back
Method
Row 1
Join in your yarn (if using a new colour) and Ch 2 (NB: this does not count as a stitch).
Make a Tr in BLO of same stitch you joined in yarn.
Continue with BLO Trs to end of row.
Join the last stitch to the top of the first Tr with a Slst (NB: not the starting Ch 2!).
Row 2
Join in new colour if using, and Ch 2.
Make a FPTr in first Tr of Row 1. This is done by YO hook, insert hook round the post of the Row 1 stitch, from the front....
....then draw a loop round the post - 3 loops on hook. Complete Tr as normal.
Make a BPTr in next Tr of Row 1. This is done as for the FPTr but insert your hook at the back.

View from above
You will see the stitch you have just worked round being pulled backwards as you complete the BPTr.

View from the front
Continue making alternating FPTr and BPTr until you reach the end of the row.
Join the last stitch to the top of the first Tr with a Slst.
Row 3
Ch 2 and make alternating FPTr and BPTr in all stitches of previous row.
You should find it easier now, as the previous stitches will be standing out to the front and back. Make sure you make a FPTr into a previous FPTr and likewise with the BPTr.
Join the last stitch to the top of the first Tr with a Slst.
Repeat Row 3 until your ribbed edge is the right length for your item. For this project, I did four rows in total. A larger item will probably need more!
Join the last stitch to the top of the first Tr with a Slst and finish off.
Using tapestry needle, sew in yarn ends.
Notes
If working in the round, make sure you have an even number of stitches for the best results.
If you change colour, do not attempt to work over the yarn ends as you go. It will be very messy and almost impossible to hide the yarn ends due to the way the alternating stitches work.
Variations
You can start an item with this ribbed edge (rather than adding it at the end). To do this, make a foundation chain for the required length, plus 2 more Ch, then work as per the Method above, starting in the third Ch from the hook.

I made a scarf to match the beanie, starting and finishing it with a deep ribbed edge
If you are working on straight rows rather than in the round, as I did for the rainbow scarf, you will turn the work at the end of each row. In this case, start even-numbered rows with a FPTr and odd-numbered rows with a BPTr, so that the ribbing pattern is maintained.

The whole scarf is crocheted with ribbing along the length, to match the deep ribbed edge on the beanie and enhance the rainbow effect
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