1 : an idea occurring later
2 : something (as a part or feature) not thought of originally : something secondary
Given this, what is an afterthought heel?
An afterthought heel is a technique where you first knit the "tube" of the sock, then go back afterwards to knit the heel. So simple, so easy! But wait... where do the heel stitches come from? Do you steek the heel? Get all crazy with the scissors?
This is where the magic happens.
You knit a section with some scrap yarn, then knit over it as usual. When you're ready to do the heel, you carefully pick out the scrap yarn, place the live stitches on needles, and there you are! Ready to knit the heel in the round.
But talk is cheap and not very helpful, so let's look at some pretty pictures! For now, I'm going to demonstrate this on a swatch. If I do this with a sock again, I'll post ACTUAL, REAL demo photos (OMFG!)
how-to: knit an afterthought heel
Step 1. Knit up to where the edge of the heel will start. Usually the heel is half of your stitches (eg. if your sock has 60 stitches, the width of the heel will be 30 stitches).
Step 2. Knit in the scrap yarn. My swatch is 20 st + 4 border stitches. I've knit 10 st with the scrap yarn.
Step 3. Transfer scrap yarn stitches back onto your left needle and pick up your main colour.
Step 4. Knit OVER the scrap yarn stitches and finish knitting your sock.
Step 5. Pick up stitches from the scrap yarn.
You have two options for doing this:
Option 1: Pick out scrap yarn and place live stitches on needles one by one
Option 2: Use needles to pick up stitches first, THEN pick out scrap yarn
I prefer Option 2, but it's really your choice. 2 reduces the chances of dropped stitches, but it can be hard to pick up the stitches, especially if you're knitting at a tight gauge. You also run the risk of picking up part of the scrap yarn. If this happens, you may have to cut the scrap yarn.
Regardless of which option you use, you'll end up with this:
(I transferred stitches onto a third needle in prep for knitting in the round)
Step 6. Knit a round, but pick up a couple stitches on each side. Increases stitch count by 4 stitches.
Step 7. Knit another round, decreasing the picked up stitches. I did [ssk k10 k2tog] x 2
Step 8. Knit as normal in the round with your normal heel decreases and you're done!