The first thing to do in preparation for making a coat is to take good, accurate measurements. Since this is more of an introduction to coat making, I’m going to simplify the measurements as best I can to make it easier for you to get started right away. All measurements should be taken over what…

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On the chest measure, I’m not sure how far into the armpit to measure. For instance, on my husband it is 15.5″ if I measure as shown on your diagram, but to get to his period shirt armseye seam is 18″. I’m thinking that 15″ is right? His shirt is the usual square-cut dropped sleeve, which looks big to our modern eye.
Yes, I’d go with the 15.5″. Just measure his actual armpit to armpit, the seams of different types of clothing can vary widely.
When measuring back length, you said “down to the bottom to where you would like your coat to be”. Where does the paletot usually fall for men in this period?
Heather, I’d recommend two or three inches above the knee. There’s no set length but that’s a good average.