If you have binding or gaping at the underarm, you may need to fine-tune the armhole fit.
Armhole Depth
The depth of the armhole is directly related to the sleeve style. The fuller the sleeve and the more dropped it is, the deeper the armhole can be without affecting comfort. In more fitted sleeves an armhole that is too low can restrict movement. The higher the underarm, the easier it is to lift your arm (1). If the garments you make typically bind at the armhole, correct the pattern before cutting the fabric.
Tissue-fit the pattern. Pin out the dart and pin the pattern together at the shoulder and side seams. Try on the tissue, matching the pattern center front and back to your center front and back. If the armhole is too low, raise it by adding tissue and reshaping the underarm curve (2). This will take some experimentation. Make the same adjustment on the underarm facing for a sleeveless garment. If the pattern has sleeves, you'll need to remove the same amount of tissue from the sleeve cap that you added to the armseye.
If you added 1/2" to the armhole at the side seam, make a 1/4" tuck across the sleeve cap, or drop the shoulder dot 1/2" and reshape the cap curve (3).
If the armhole is too high and the garment has a set-in sleeve, the fit can be adjusted as you sew.
Set the sleeve into the armhole as usual and trim the seam allowances to 1/4" from notch to notch under the arm. Try on the garment. If the armhole is too tight, stitch 1/4" lower at the underarm, tapering back to the original stitching at the front and back notches (4). Re-trim the underarm seam allowance to 1/4", and try on the garment again.
If it's still too tight, stitch an additional 1/4" lower in the same manner. Trim and try on.
For sleeveless garments, lower the cutting line and seamline 1/4" before cutting. Then adjust the facings to match (5). If the garment is still too snug under the arm, stitch an additional 1/4" lower and trim as directed for a garment with set-in sleeves.