Look to see how to do a back post double crochet

https://newstitchaday.com/bpdc-back-post-double-crochet/

It's a double crochet but goes around the post instead of in the top of the stitch.

How to Work Front Post Double Crochet Stitches dummies

Lots of free info on the internet:

How to Crochet the Double Picot String Network Stitch

How to Crochet Picot Stitch

Picot Double Single Crochet Stitch Tutorial Girlybunches

No, a HDC is in-between a single crochet and a double crochet.

double crochet in the front post


crochet dcfp Google Search

It can be done, but would be very difficult to explain.

In effect, when you get to what would be a corner, you add extra stitches to what would be the corner. You can add some chain stitches. Another thing would be (for example) if doing single crochet, go from single crochet, to double crochet, to triple crochet, then to double crochet, and back to single crochet. The extra length of the double and triple crochets would form a corner that will become more evident when the next round is crocheted.

How to Crochet Single Crochet Evenly Around Square

crochet corners Google Search

This video will probably help:

Crochet Front Post Treble Crochet FPtrc

To make the stitch on the back of the stitch, you would insert the hook from the back side of the project and work the treble stitch on the back.

How to Crochet Back Post Triple Treble Crochet Stitch

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try these link for videos
and websites
crochet stitch dc2tog Bing Videos Double crochet two together dc2tog Crochet Stitch Glossary Crochet Me How To Make Double Crochet Decrease dc2tog