Skip this if you are not interested in minutiae of shirt measurement.
Now and then I am asked to measure a shirt like we measure a suit; shoulder seam to shoulder seam, and shoulder seam to end of sleeve. I have already provided actual collar and sleeve measurements (measured the traditional way) and usually have provided P2P. In my experience, if a shirt has the correct neck-sleeve size for me then the shoulder seam falls at the right spot. That is 100% of the time.
I know that the shoulder seam to end of sleeve measurement is unnecessary. If the sleeve length in the collar-sleeve size format is your sleeve length then the end of the sleeve will be in the right place at your wrist.
The question is whether the shoulder seam to shoulder seam measurement is of any use to anyone. Do any of you find that some OTR shirts which are of the correct size (collar-sleeve actual measurements) actually do not fit you because the shoulder seam to shoulder seam width is too wide or too narrow?
I actually find the 16-33 slim fits and extra slim fits that my neck and arms need from Brooks Brothers are slightly too wide in the shoulder. Maybe 1/4"-1/2" on each side. But so far it does not bother me enough to do anything about it.