the.com/armhole
The garment's polite invitation for your arm to enter the building.
means The opening in a shirt or jacket where the sleeve meets the body and your arm passes through.
from A plainspoken English compound from arm plus hole, in use since the 1400s — Middle English saw no need for fancier tailoring vocabulary.
Tailor termPros call it the armscye, from arm plus eye.
Fit make-or-breakToo tight binds, too low gapes embarrassingly.
Body doubleEach garment naturally needs exactly two.