How to lengthen/shorten an armhole height?

There are two possible scenarios. In the first case, the bust pulls under the arms. There is not enough ease at the armhole. In the second case, the garment tends to ride up when the arm is raised. The underarm point is too low.

Definitions

Let's first start by defining two important terms discussed in this tutorial.

Armhole : curved line between the underarm point and the shoulder slope, on the bodice part (front and back).

Sleeve head : “bridge” shaped outline on the upper part of the sleeve.

Basic material to perform a transformation

- Adhesive tape
- A parrot rule
- A square
- A graduated ruler
- A support sheet
- A pair of paper scissors
- A fine-tipped pencil/criterion

Changes to be made

In both cases, the method is identical in order to resolve the problem encountered.
You will need to cut the pattern pieces in 2 and add or subtract the desired value between these 2 parts.
First, start by increasing or decreasing the armhole height on the front and back pieces. Then modify the sleeve head by adding or removing the same amount as on the bodice.

For more details on the method discussed below, refer to method A in the article Basic techniques for shortening or lengthening a pattern .

• Draw an axis perpendicular to the straight grain on the front and back. Place it anywhere between the underarm point and the shoulder slope. Mark this line preferably at the same height on both pieces.
• Cut on the line, then spread the pieces apart to increase the armhole height or overlap the pieces to reduce the armhole height.
• Then draw an axis perpendicular to the straight grain on the sleeve head. Place it anywhere between the underarm line and the top of the piece. Cut, then spread or overlap in the same way as on the bodice.

Case of a simple bodice: The Chalk blouse

Case of a more complex bodice with cutouts: The Maïa blouse

Line checks

Once the lines have been traced, compare the old line and the new one by measuring the assembly lines. Normally, the sleeve head is slightly longer than the addition of the front and back armholes. This excess must be absorbed at the time of assembly. It is important that the new sleeve head keeps the same excess value in order to remain faithful to the cut of the garment.
If the outline of the headstock is not suitable, adjust its height until a good balance is found.

Example

Measurements on the basic pattern still unmodified:

• Front armhole: XX cm
• Back armhole: XX cm
• Total armhole measurement: XX + XX = XX cm
• Sleeve head: XX cm
• Value of the surplus on the head of the sleeve: XX – XX = XX cm

Comparison of the surplus value found on the modified and unmodified sleeve head:

• If the value is equivalent to within 0.5 cm. No modification is necessary
• If the value is more than 0.5 cm higher, then the sleeve head height will need to be slightly reduced.
• If the value is lower by more than 0.5 cm, then the sleeve head height will need to be increased.