The afterthought heel is added after the entire body of the sock is completed down to the toes. You can use it instead of a gusset heel. It is easy to replace once worn out; it also lends itself to the use of contrasting yarns for an interesting heel addition.
At the end of the leg, knit half of the stitches using a smooth waste yarn of a similar thickness to the working yarn.

Your waste yarn can be a different fiber composition, so long as it’s smooth (not sticky) and of an appropriate thickness. You can even use the same yarn as you’re knitting with, but make sure it’s a different color!
Slip these stitches back to the left needle without twisting. Re-knit these stitches in the working yarn and continue working in pattern straight to the toe.
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Once the toe is complete, you should recover the stitches for the heel.
Remove the waste yarn, placing the live stitches onto two double-pointed needles.
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The heel opening will have the same number of stitches as the sock body.
Re-arrange the stitches onto four needles
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Place markers at each side edge of the heel opening to mark the decrease points.
Decrease for the heel exactly as for the basic round toe.

End the decreases when approximately 2 inches’ worth of stitches remain.
Graft together these stitches using the Kitchener Stitch.
This is the bottom corner of the heel.