A keyhole opening is a short slit, usually 8 to 12 cm (3 to 4 3/4 in) long at the center back of a neckline, that lets a close neckline pass over the head and then closes with a button and a loop at the top. The reliable way to sew it is with a facing: mark the slit on the interfaced facing, sew a narrow V around it with the loop caught in the seam at the top corner, cut down the middle, turn the facing to the inside, press, then sew the button opposite the loop. It takes a scrap of self fabric for the loop, one small button and about half an hour.

What is a keyhole opening and when do you need one?

A keyhole opening solves one problem: a neckline that is too small to go over the head without a zip. An adult head measures around 56 cm (22 in) around, and a neckline needs a few centimeters more than that to pass comfortably. A jewel neckline on a woven top or a shift dress is often 50 to 54 cm (19 3/4 to 21 1/4 in) around, so the slit adds the missing length twice over, once on each side.

You will find it on back necklines of blouses, shift dresses and children's clothes, and sometimes at the front as a design detail. The same construction, scaled up, is the teardrop opening of a summer top.

What do you need?

  • The garment piece with the opening, usually the back, and its neckline facing. The facing must be deep enough to extend at least 2 cm (3/4 in) below the bottom of the slit.
  • Lightweight fusible interfacing for the facing.
  • One button of 1 to 1.2 cm (3/8 to 1/2 in). A shank or a ball button sits best in a loop.
  • For a rouleau loop: a bias strip of self fabric 2.5 cm (1 in) wide and 10 cm (4 in) long, plus a loop turner or a bodkin. For a thread loop: a needle and buttonhole or topstitching thread.
  • A water-soluble pen, fine pins, small sharp scissors and a point turner.

How do you sew a keyhole opening with a facing, step by step?

  1. Interface and mark the facing. Fuse the interfacing, then draw the center back line on the wrong side of the facing. Mark the bottom of the slit 10 cm (4 in) below the neckline seam line. Draw the stitching lines as a long narrow V: 6 mm (1/4 in) apart where they meet the neckline seam line, tapering to a point at the bottom mark. Draw the point as one tiny stitch across, not a sharp angle, so the fabric has somewhere to turn.
  2. Make the loop (see the next section) and pin it to the right side of the garment at the top of the slit, on the side where you want the loop. The loop itself lies flat on the garment, pointing away from the slit, and its two ends cross the stitching line into the narrow strip that will be cut away. Baste the ends in place 3 mm (1/8 in) from the edge.
  3. Pin the facing to the garment, right sides together, matching the center back and the shoulder seams. Sew the neckline seam at 1.5 cm (5/8 in), stopping 6 mm before the slit on each side.
  4. Stitch the slit. Shorten the stitch to 1.5 mm (about 16 stitches per inch). Start at the neckline on one side, sew down the V to the point, take one stitch across, and sew back up the other side to the neckline. The short stitches keep the point from fraying open when you cut.
  5. Cut. Cut down the center of the V, stopping 1 to 2 mm from the point. Trim the neckline seam allowance to 6 mm (1/4 in), clip the curve, and trim the loop ends level with the cut edge.
  6. Turn and press. Push the facing through to the inside, using a point turner in the bottom of the slit. Understitch the neckline seam, pressing the facing and allowances away from the garment and stitching 2 mm from the seam; the slit is too narrow to understitch, so roll its edges by hand with the seam 1 mm to the inside and press.
  7. Secure the facing. Tack the facing to the shoulder seam allowances by hand or by stitching in the ditch at the shoulders.
  8. Sew the button on the other side, 1 cm (3/8 in) in from the finished edge, so the two edges meet without overlapping when the loop is on.
Tip: Sew a test on a scrap of the garment fabric with its facing before you cut into the back. The V width and the stitch length depend on how much the fabric frays.

How do you make the button loop?

A rouleau loop is a narrow tube of bias fabric. Fold the 2.5 cm (1 in) bias strip in half lengthwise, right sides together, and stitch 5 mm (3/16 in) from the fold, stretching the strip gently as you sew so the stitches do not snap later. Trim the seam allowance to 3 mm, then turn the tube with a loop turner. A loop for a 1.2 cm button needs about 4.5 cm (1 3/4 in) of tube: twice the diameter plus the thickness, plus the ends caught in the seam. Check the length around the button before you baste it in place.

A thread loop is quicker on fine fabrics and suits a hand-finished blouse. After the opening is finished, make four to six strands of thread between two points 1.5 cm (5/8 in) apart at the top of the slit, then cover the strands with close blanket stitches. The same stitches are described in the buttonholes and buttons guide. A third option is a 5 cm (2 in) length of 2 mm round elastic, which forgives a loop that came out a little short.

Common mistakes when sewing a keyhole opening

  • A slit that is too short. Measure the neckline plus twice the slit on the pattern before you sew. If the total is under 60 cm (23 1/2 in), lengthen the slit.
  • A wide V at the point. Lines further apart than 6 mm at the top give a visible gap; a V that is still wide at the bottom gives a bulky, rounded end instead of a clean point.
  • Long stitches at the point. With a 2.5 mm stitch the fabric frays past the stitching the first time the slit is turned. Shorten to 1.5 mm for the whole V.
  • Cutting past the point. Stop 1 to 2 mm short. If you do cut too far, a dot of fray stopper on the wrong side saves it.
  • Skipping the interfacing. The facing then stretches as you sew and the slit edges ripple.
  • A loop that faces the wrong way. Before you stitch the facing, fold the loop over the stitching line in your head: after turning, whatever lay on the garment side will stick out past the edge.
  • A facing that rolls to the outside. Understitching and a 1 mm roll to the inside fix most of it; see how to keep a facing from rolling out for the rest.

When should you use a keyhole instead of a placket or a zip?

Choose a keyhole when the neckline almost fits over the head and the garment is loose enough below it. It is the lightest closure there is: no zip tape, no overlap, one button. It suits the Sewly woven top and shift dress with a round neckline, and any blouse in lawn, voile or crepe.

Choose a placket when the opening must be longer than about 15 cm (6 in) or carry several buttons; a placket has an overlap and an underlap and stays closed in wear. Choose an invisible zip when the bodice is fitted through the waist, because a keyhole only opens the neckline, not the body of the garment. On sheer or very fine fabric, replace the facing with a narrow bias binding around the slit and neckline so no facing shadow shows through.