A neckline gapes because its edge is longer than the body it rests on, so it stands away from the chest instead of lying against it. The fix on a pattern is to pinch out the extra length on a toile as a small wedge from the neckline toward the bust point, transfer that wedge to the pattern as a dart, and rotate it into the existing bust or waist dart or a seam, so the neckline gets shorter without a visible dart. On a garment that is already sewn, the edge can be shortened by taking it in at the shoulder seams, by a hidden dart at the shoulder, or by running elastic or a shorter binding inside the edge.

Why does a neckline gape?

A neckline is a curve cut through the hollow of the chest, and that hollow changes shape from one body to the next. A deep V or a wide scoop crosses the slope between the collarbone and the bust, and if the pattern assumed a fuller chest there than yours, the edge has nowhere to rest. The usual causes:

  • The neckline is cut for a fuller high chest than yours, so the edge is longer than the body between the shoulders and the bust.
  • The bust is larger than the pattern size and pulls the center front down, which drags the neckline open at the sides. Here the neckline is a symptom of a full bust adjustment that is still to come.
  • The shoulders are narrower than the pattern, so the neckline starts too far out at the shoulder seam.
  • The edge was stretched during sewing. Most of a curved neckline runs on the bias, and handling it without a row of staystitching lets it grow by 1 cm (3/8 in) or more.
  • A rounded upper back or a forward head opens the back neckline rather than the front.

Fitting fixes the first three; sewing habits fix the fourth. Tell them apart by measuring the toile's neckline against the pattern: if the fabric edge is longer than the paper, the edge stretched.

How do you find out how much the neckline gapes?

Sew a toile of the front and back bodice, with the neckline at the stitching line, and put it on over the underwear you will wear with the garment. Stand in front of a mirror and pinch the extra fabric at the neckline edge into a wedge that points toward the bust point, with the fold running down the chest. Pin the wedge and move around: look down, reach forward, sit. The neckline should now lie flat without the fabric below it pulling.

Measure the pinch at the edge with a ruler: the whole width of the fold, both layers together. Typical results are 6 mm to 2 cm (1/4 to 3/4 in) on a V or scoop neckline. Do the same on the other side; a difference of a few millimeters between the two is normal and the pattern takes the average. Mark where the wedge meets the neckline. Making a toile shows how to mark the toile and transfer the pins.

If pinching at the neckline does not flatten it but taking the shoulder seams in at the neck does, the neckline is the right length and sits too wide: narrow it at the shoulder instead, which is the simpler alteration covered at the end.

How do you remove the gape on the pattern?

The wedge you pinned is a dart. Transfer it to the pattern, then move it somewhere it will not be seen.

  1. Draw the wedge on the pattern. Mark the point on the neckline stitching line where the pinch sat, and draw two legs from the bust point to the neckline, the measured distance apart at the edge, 1.5 cm (5/8 in) in our example.
  2. Draw a line from the bust point to the existing dart, a side bust dart or a waist dart, or to the center of a princess seam on a princess bodice.
  3. Cut the neckline wedge along both legs up to, but not through, the bust point. Cut the line to the existing dart up to the bust point as well, leaving a paper hinge.
  4. Close the neckline wedge by bringing its two legs together and taping. The cut toward the existing dart opens by the same amount, making that dart wider.
  5. Redraw the neckline as a smooth curve across the closed wedge; it is now shorter by 1.5 cm on each side. Redraw the legs of the widened dart from the dart point to the new ends, and refold the dart to true the seam line.

On a bodice with no darts, a knit top, a slip dress or a boxy blouse, you cannot rotate the wedge away, but you can still remove it. Close the neckline wedge and let the paper open at the hem: the hem widens a touch, which disappears into the seam allowance or a slight flare. Or rotate it into the armhole on a sleeveless top, where it becomes a small, invisible amount of extra ease.

Alter the facing, binding or lining by the same amount in the same place, so it matches the new edge.

Tip: Shortening the neckline on the pattern changes how the collar, if any, fits. Measure the new neckline seam and adjust the collar length to match, usually by taking the same amount out of the collar at the shoulder notches.

How do you stop a neckline from gaping while you sew?

Even a neckline that fits on paper can be sewn into a gape. Three habits prevent it:

  • Staystitch the neckline within an hour of cutting, 1.2 cm (1/2 in) from the raw edge, sewing from each shoulder toward the center front so the two sides are treated the same. What staystitching is and why it is sewn in that direction.
  • Cut the facing or binding slightly shorter. A neckline facing cut 6 mm (1/4 in) shorter at each shoulder, or a bias binding cut 1–2 cm (3/8–3/4 in) shorter than the edge, draws the neckline in as the edge is eased onto it. Finishing necklines with bias binding shows how to distribute the ease around the curve.
  • On knits, use clear elastic in the neckline seam, cut 2–3 cm (3/4–1 1/4 in) shorter than the edge and stretched to fit as you sew, so the neckline recovers after every wear.

Understitching the facing afterward keeps it from rolling to the outside, which otherwise looks like a gape even when the length is right.

Can you fix a neckline that has already been sewn?

Yes, with more or less visible results:

  • Take in the shoulder seams at the neck. Open the facing at the shoulder, resew the shoulder seam 6 mm to 1 cm (1/4 to 3/8 in) deeper at the neckline end tapering to nothing at the armhole, and close the facing. This shortens the neckline by twice the amount and suits a scoop or a boat neck.
  • Hide a dart in the shoulder seam. On a wide neckline, a small dart from the shoulder seam toward the bust, 1 cm wide at the seam and 8 cm (3 in) long, reads as part of the seam.
  • Thread elastic through the facing. Open the facing's inner edge for a few centimeters, run a length of 6 mm (1/4 in) elastic cut 2–3 cm shorter than the neckline inside it, secure both ends and close. The gentle gather brings the edge in and is invisible from the outside.
  • A tuck at the center front of a V neckline, 6 mm deep, sewn as a tiny pleat at the point of the V, works on a casual top when nothing else does.

For a back neckline that gapes, the same methods apply at the back shoulder, or the excess is taken into the center back seam if there is one, tapering out over 15 cm (6 in).

Does a made-to-measure pattern gape less?

A pattern drafted from your shoulder and bust measurements starts from a neckline sized for the body you have, rather than for a chart that puts the bust and shoulders together in one size. Sewly drafts that way, which removes most of the gape that comes from a size mismatch. The hollow of the chest is not something a tape measure captures, so a deep V or a wide scoop still deserves a quick toile before you cut. How to fix gaping armholes uses the same pinch-and-rotate method on the other edge that tends to stand away.