How to lengthen/shorten an armhole height?

Two cases can occur. In the first case, the bust pulls under the arms. The ease is not important enough at the level of the armhole. In the second case, the garment tends to rise when the arm is raised. The underarm stitch is too low. 

Definitions

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

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

Head of sleeve A "bridge" pattern on the upper part of the sleeve.

Basic equipment to perform a transformation

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

Changes to be made

In both cases, the method is the same to solve the problem.
You will have to cut the pattern pieces in 2 and add or subtract the desired value between these 2 parts .
Start by increasing or decreasing the armhole height on the front and back pieces. Then modify the sleeve head by adding or subtracting the same value as on the bodice.

For more details on the method described below, please refer to method A in the article Basic Techniques for Shortening and Lengthening Patterns.

• Draw a line perpendicular to the straight thread 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 to increase the armhole height or layer the pieces to reduce the armhole height.
• Then draw a line perpendicular to the straight line on the headstock. Place it anywhere between the underarm line and the top of the piece. Cut, then spread or overlap in the same manner as on the bodice.

Case of a simple bodice : The blouse Craie

 

Case of a more complex bodice with cut-outs : The Maïa blouse

 

Line checks

Once the lines have been traced, compare the old and the new layout by measuring the joining lines. Normally, the sleeve head is slightly longer than the addition of the front and back armholes. This excess should be taken care of during assembly. It is important that the new sleeve head keeps the same amount of excess in order to stay true to the cut of the garment.
If the sleeve head is not the right fit, adjust its height until you find a good balance.

Example

Measurements on the still unmodified base pattern:

• Armhole front: XX cm
• Armhole back : XX cm
• Total armhole measurement: XX + XX = XX cm
• Head of the handle : XX cm
• Value of the surplus on the head of the handle: XX - XX = XX cm

Comparison of the surplus value found on the modified and unmodified headstock:

• If the value is equivalent to within 0.5 cm. There is no need to make a modification
• If the value is more than 0.5 cm higher, then you will have to slightly reduce the height of the head.
• If the value is more than 0.5 cm lower, then the height of the headstock must be increased.