What the Heel Flap Is and Why It's Reinforced
The heel flap is a rectangular section of flat knitting worked back and forth on half your sock stitches. It forms the back of the heel — the part that takes the most abrasion inside a shoe. Because it's worked in a specific slip-stitch pattern, the fabric ends up roughly twice as dense as ordinary stockinette: each slip stitch carries a bar of yarn across the surface on the wrong side, and those bars create a subtle cushion that wears far longer than plain fabric would.
The heel flap stitches are typically the stitches that were at the back of the sock — if you're working on DPNs, it's usually needles 3 and 4 (or the rear half of the sock on a magic loop or two circulars). You'll work back and forth on these stitches alone while the instep stitches rest.
The Slip-Stitch Heel Flap Pattern
The classic heel stitch for 32 stitches:
Right side rows: Slip 1 purlwise with yarn in back, knit 1, slip 1 purlwise with yarn in back; repeat from to end.
Wrong side rows: Slip 1 purlwise with yarn in front, purl to end.
The first stitch of every row is slipped — this creates a clean chain of edge stitches along both sides of the heel flap, and those edge chains are what you'll pick up during the gusset phase. Each pair of rows creates one chain stitch. For a 32-stitch heel, you'll work approximately 32 rows (16 chain stitches per side), making the flap roughly square.
Count your chain stitches on one side as you go — they're easy to see and a reliable way to know when the flap is done. When the flap is square (or slightly longer than wide for a deeper heel cup), you're ready to turn.
The Heel Turn: What It Does
The heel turn is short-row shaping that transforms a flat rectangle into a three-dimensional cup. You use decreases to close in from both sides, building a curved base for the heel. The resulting shape is what lets your foot sit inside a sock rather than the sock hanging straight off your leg like a tube.
The math behind the heel turn is precise and consistent: you work to the center, work a decrease, work one more stitch, turn, work to the center plus one, work a decrease, work one more stitch, turn — and repeat, working one stitch further past the center each time until all the outside stitches have been consumed by the decreases.
How to Work the Heel Turn: Step by Step
Starting with 32 heel flap stitches (adjust proportionally for other counts):
Set-up row (RS): Knit 18, ssk, k1, turn. You've left 11 stitches unworked at the end.
Row 1 (WS): Slip 1, p5, p2tog, p1, turn. You've left 11 stitches unworked at the end.
Row 2 (RS): Slip 1, k6, ssk, k1, turn.
Row 3 (WS): Slip 1, p7, p2tog, p1, turn.
Continue in this pattern: each row, you work one stitch further than the previous row, then work a decrease (ssk on RS, p2tog on WS), then k1 or p1, then turn. The "turn" leaves a small gap in the fabric — that gap is your guide for the next row's decrease position. You can see it clearly; knit or purl to the stitch just before the gap, decrease across the gap, and work one more stitch.
Continue until all stitches have been consumed. You'll end with approximately 18 stitches remaining on the needle (or half your heel stitch count plus 2). Both sides will have used up all the waiting stitches. The last row should be a right-side row.
Picking Up the Gusset Stitches
Now you rejoin to work in the round and add the gusset stitches along both sides of the heel flap. This is where those chain edge stitches come in.
With the right side facing, knit across the heel stitches (the ones remaining after the turn). Now, along the right side of the heel flap, pick up one stitch from each chain edge stitch — insert the needle under both strands of the chain, wrap yarn, and pull through. Pick up 16 stitches on a standard flap (one per chain stitch). Work across the instep stitches. Pick up 16 stitches down the left side of the heel flap. You're now working in the round again with more stitches than your original count.
To prevent holes at the junction between heel and instep (a common problem), pick up an extra stitch at each corner — between the last heel stitch and the first instep stitch, and between the last instep stitch and the first heel stitch. On the next round, decrease these corner stitches away by knitting them together with their neighbours. This closes the gap that forms naturally at the gusset join.
Starting the Gusset Decrease Rounds
Decrease round: Work to 3 stitches before the end of the heel needle, k2tog, k1. Knit all instep stitches. K1, ssk at the start of the heel needle. Knit to end.
Plain round: Knit all stitches.
Alternate these two rounds until you're back to your original stitch count. Then knit the foot to the desired length, and work your chosen toe.