|I thought of adding this tip since prom season will be coming up.
Many people, especically those with narrow shoulders, need a smaller neckline size than what is on the pattern, especially if the design is a wide neckline close to the edge of the shoulder. Typical narrow shoulder alterations tell you to sew the armhole seam further in. Trouble is when you do that there's nothing left. The whole shoulder area has to be cut out so it's closer to the neck.
I like to use a multisize pattern where the sizes are lined up on the centerline. Trace the smallest size neckline and cut out to use as a template. At each pattern point corner you estimate where the next smaller size should be and then put in your dots. Use your cutout template to use as a guide in drawing the lines. Trace a few inches of the centerline, the neckline, and the armhole down to the notch. At the armhole notch blend into the size you have determined to be the best for bustline fit.
I'm kind of spatially impaired-with those narrow necklines and seam allowances attached it's kind of hard to visualize the final result, so I usually do a neckline paper test. Trace just the upper portion of the bodice to the armhole, both sides, front and back and tape together at centers. Trim off the neckline seam allowances and the armhole seam allowances and see how it looks on the person. Take into consideration that if the neckline is bias and/or the fabric is stretchy you may want to go down one more size, even if the paper pattern seems to fit well.
Each neckline size decrease will also decrease the shoulder to armhole depth, so you may need to adjust that during the paper fitting.
If you have the book Fit for Real People, refer to pp 34-43. I think if I were making those dresses, I would have tried this technique for neckline alteration. I think this is common with young girls that are just developing-their bustline develops first and it takes awhile for their neck and shoulders to fill out.
A person can use a variety of sizes on different parts of the body. I found this out the hard way when I made a bridesmaid dress for my daughter- she was size 2-4 in the neckline, size 6 at the bust, size 10 at waist and hip, and size 12 at bicep.