It's a rare thing when artists show us any seam lines, and we have to be grateful when they do.
Here are some examples of seams.
A center back seam, arm seams, and (perhaps) and armpit gusset from the 1330s frescoes in Sienna.
What looks like an open front seam from a
Tacuinum Sanitatis.
A shoulder seam and an armscye seam from the Sienna frescoes
An inset gore from a 13th C (?) Spanish image. The extant St. Louis shirt (also 13th C) has these as well.
Armpit gussets and side seam from a 15th C. Italian scene of laundry drying.
Another open front; suggesting a center front seam. From a 1460s French book of hours.
A shoulder seam and an armscye seam (well off the shoulder) by Dirk Bouts in 1458
A center back seam from a 15th C French Froisart's Chronicles.
A shoulder seam, armscye seam, a side seam, and an armpit gusset from a
tacuinum sanitatis from the turn of the 14/15 C. I cant tell is that's supposed to be a gore or a vent at the lower end of the side seam.
Armscye seams which lead down to points in the side seams. I suspect that there were armpit gussets here as well. One of the shirts is open in front, suggesting a front seam. This is again from the Sienese governmental allegory frescoes.
An open front; suggesting a front seam. From a 1440s Crucifixion scene.
Shoulder seam, side seam (with gusset?) and vent or gore. From a
tacuinum sanitatis.
A shoulder seam and an armscye seam from a 15th C book of hours (?)
A shoulder seam and an armscye seam from a
tacuinum sanitatis.
A suggestion of a center back seam from a book of hours of C 1500
Here's a shoulder seam, and an armscye seam. There is also a horizontal line at the level of the armpit that may tell us something important. The shallow "V" of the back neckline suggests a back seam as well.
I'll edit more into this if/when I find more.
Mac