Well, here’s a 15th century undershirt. It’s not based on any extant pieces, but rather period illustrations.
This garment takes an incredibly small amount of fabric. 2 yards of 60″ wide material is all you need for a person with a chest less than 55″ around. An additional ¼ yd is all you need to fit a man up to 80″ around! Look at the figures below for two ways to layout your fabric.
Measure the wearer’s shoulders across the back with the back expanded (flexed). Add two inches to this measurement and it should be enough. If you’re unsure, make a
muslin and test it first. Call this measurement “A”.
Now measure from the shoulder to the wrist around the bent elbow. Call that “C”.Now measure the recipient from neck to mid-thigh. Call that “B”.
Cut two pieces A wide and B long. Hold a ruler up the the wearer’s neck and mark how wide it is linearly. Call this “D”.
Fold the AB piece in half widthwise and make marks ½D” from the center fold. Unfold the fabric and sew from the outside edge to those marks. This is your shoulder seam. The slit left is the beginning of the neck hole. I then slice down the center of the front vertically about 6″ and make this the opening. You may want to cut a more rounded opening and hem it though.
Next cut the sleeves C long by about 18″ wide. This width can be made smaller for tighter sleeves, but it is better to start out with something too big than too small.
Mark the center of the width (9″) and attach that to the shoulder seam of the body pieces. Sew the one side onto the front and the other onto the back. Don’t sew the bottoms of the sleeves yet.
Cut two perfect squares, 6″ x 6″. Insert a corner into the corner where the sleeve meets the body. Sew the top the the sleeve and the side to the front of the body. Turn the garment over and sew the outside side to the other edge of the sleeve and the bottom to the back part of the body. The square should now be folded into a rectangle. Repeat this on the other side.
Sew up the sleeve bottoms, hem the bottom and sleeves, finish the edges and you’re done! There’s your 15th century undershirt.
It makes a nice finish if you french seam the entire garment.