If It Ain't Broke...
Yesterday, I got a help request from a seamstress working on one of my patterns. "The fronts and backs don't line up right and the sleeve doesn't fit nicely in the armscye. If I force the sleeve into the armscye, it makes goofy wrinkles at the shoulder when the wearer puts his arms down."
Guess what. That's exactly where there are goofy wrinkles on the original garment.
As modern people who are trying to replicate period clothing, the most difficult part of our task is to discard our modern sense of the way things should look.
Just the other day, my friend was complaining to me that she couldn't get the 15th century shirt she's making her boyfriend fit right in the shoulder, so she wanted to break with historical construction and add a shoulder seam.
"Shoulder seam?" quoth I. "You don't need no stinkin' shoulder seam!"
My friend is a professional theatrical costumer and an amazing modern tailor. People pay top dollar for her custom-made historic costumes and she's worked for many movies, TV shows, and Broadway shows.
But she's not a clothing historian (at least not for the medieval period). So she wants to put in a shoulder seam because "it looks wrong without it."
It's not wrong. It's medieval.
You see, in the medieval and Renaissance periods, people had to make use of every scrap of fabric. Labour was cheap and fabric was expensive. In the early modern period that followed, wealthy people wore lace collars that would equate in price to an estate in Ireland. So you see where I'm going with this. If you had an expensive piece of brocade, it probably cost more than your house. So when you made something from it, you pieced it and turned it and used every last scrap.
Modern dressmaking techniques waste a lot of fabric. They are engineered for speed of sewing. In the periods we study, speed was not as important as the conservation of fabric.
With shirts and simple garments like that, the size of the wearer didn't dictate the size of the shirt. The size of the linen did! So you should have sleeve seams that slip off the shoulder. You should have a funny wrinkle on the top of the shoulder. This allows you to maximize the use of your linen and throw nothing away. Curved seams fit the human body better, but they waste a lot of fabric.
So relax. And think medieval.