(In these photos, I'm working 2 strands of green yarn together as one, to make my 30-minute hat, so please keep this in mind when looking at it.)
Both of these stitches MUST BEGIN with a row/round of regular dc sts prior to starting the row/rnd that is fpdc/bpdc.
Front Post Double Crochet (FPDC):
This stitch is essentially a plain dc st, but instead of putting your hook into the stitch as you normally would, you put it behind the "post" of the dc directly below that st.
|Insert your Hook behind the "post" of the dc that is directly below where you would normally make your dc stitch.|
|Yarn over, as with a normal dc.|
|Pull the yarn through, behind the "post" of the dc. This pushes the stitch from the previous row/rnd to the front of your work also.|
|As with your normal dc, yo again, and complete the dc st as normal. You may want to do this a little loosely until you get used to doing fpdc sts, as they tend to be bulkier sts in general.|
Back Post Double Crochet (BPDC):
This stitch is the inverse of the FPDC. Again, it is a dc st that is essentially worked around the post of the dc below where you would normally put your st. This time, however, you will be putting your hook in front of that stitch, pushing it to the back of the work.
|Insert your hook from the back of your work to the front, to the right of the dc that you're going to work with. This should be the "post" that lies directly below the st where you would normally put your st.|
|Push the hook over the post of that dc, and to then the back of your work (using the space to the left of the dc's post). In this photo, the post of the dc is hidden behind the hook, as it should be with your work|
|Yarn over, and pull the loop all the way through to the front, so that you're left with 3 loops on your hook. This pushes the dc st from the previous row/rnd to the back of your work.|
|Yarn over and complete the dc st as you normally would.|
Alternating your FPDC and BPDC sts can create a ribbed effect. It also tightens up your stitches, while at the same time adding a stretchy quality, so its great for the bottom of hats or the tops of socks/leggings. Some patterns will have you add FPDC sts only, and this creates a line of ribbing that is on the front side of your material only.
If you are new to crochet, and trying to learn many stitches, I HIGHLY recommend you buy the book "Stitch and Bitch Crochet: The Happy Hooker". It has great instructions with pictures on how to do almost every stitch you'll ever need, along with how to read patterns, how to read stitch grids, how to troubleshoot your work, and some basic patterns to help you get started. No, I'm not associated with the book/publishers/author in any way. I just love the book, and all the crochetters in my local craft group reference it constantly.