Here’s your 40 second visual rundown on just what an underarm gusset does in a simple smock/shift/chemise/shirt pattern. I have two very simple smocks here – one eleventh century style, a la Kohler, and the other more of a sixteenth century style, adapted from Arnold. There’s a drastic difference in the fit at the shoulder.
The Kohler smock is not quite a T-tunic cut – it actually sets the sleeve at a slight angle to add some extra fabric for movement under the arm. The second smock uses a relatively large underarm gusset. (It’s a faked gusset, mind you, because the 1″ square gusset I was trying to set defeated me. It achieves the correct shape.)
By adding the gusset, we add a lot more ease under the arm. That prevents the smock from bunching around the arm when the arm is lowered, as well as giving us a much wider range of motion. (The dolly on the right could play basketball in her little smock, while the other would have to be careful how she tried to move her arms.)