Here we are at last - the sock trick, or, how to avoid holes!
This is for a sock using the standard heel flap construction, worked from the top down. And I'm going to apologise in advance for my photographic technique, or rather my lack thereof.
Here I am partway through picking up stitches along the first side of the heel flap. I have worked slip stitch edges - essential for this method - and I am picking up both loops of the slipped stitches.
Like that. The worked stitches happen to be in blue, the double loop of the stitch I'm working on is in red/yellow/black.
Now, if you look at that column of slipped stitches going down the side of the heel flap, you'll see that it runs into a column of stitches in the fabric of the sock.
When you get to the bottom of the heel flap, and you've picked up all the slipped stitches, you want to pick up one more stitch so that there won't be a hole.
And this is the key.
You pick up the next stitch in the column.
That one. I've put a dpn through it so that you can see which one I mean. Both sides of the stitch get picked up, exactly as you did for the slipped stitches.
It is the first one below the joining thread, at the base of the heel flap. Can you see that thread, going across from the base of the heel flap, over to the needle where the instep stitches are waiting? The stitch just below that thread is the one you want.
So, pick up the two loops of that stitch, and knit through them together. This gives you one extra stitch on your right hand needle.
Work across the instep stitches as usual, and then you have to deal with the corner at the other side of the heel flap.
Once again, look at the chain of slipped stitches down the side of the heel flap. They continue into a column of stitches in the sock fabric - this column -
I'm just pointing with the dpn here. You can see the the column to the right of that goes into the instep stitches, which have been worked in blue. The stitch you are after here is the first one in the column I'm pointing at, just below the thread that goes across between the instep stitches and the heel flap.
This stitch is the one. I've put a dpn through it, to make it easier to work - this gets knitted, exactly like the slipped stitches along the side of the heel flap.
Don't worry about losing the extra stitches on the next row or anything like that - just decrease them away as part of the usual gusset decreases. You've just got an extra stitch each side in the gusset, that's all.
And that's it! Knit on, as they say.......