The letterman, also known as the varsity jacket, has its roots in American college baseball. Originating from the Harvard baseball team in 1865, the idea of sewing the first letter of the school onto a uniform would soon bleed over to the Harvard football team. Although this was more of a team uniform, these origins set the trajectory for what we now know as the letterman jacket.
Since the cardigan was gaining popularity over the pullover, the letter had to be moved to the left from the center. The popularity of this “letterman” jacket eventually spread across the country and the jacket that we know today took its form in 1930.