Much like the shift, this undershirt was meant to be worn underneath all layers of clothing. The diamond shape shown is a gusset in the armpit, and extends the range of motion in the garment. Undershirts like this one were worn by all classes of people, and therefore needed to be suitable for a wide range of tasks. It is a truly versatile garment in this respect. There is a common belief that the clothing worn in the past was restrictive and limiting, but this dispels that belief. Clothing was made to fit the body. Now, we expect bodies to change in order to fit clothing.
“Clothing could only be practical. Especially clothing that the working classes were wearing, it had to be functional, they had to be able to do things that they had to do to survive in their clothes.” -Mackenzie Broders