Measuring Armhole Depth

Armhole Depth is a critical measurement and you want to be sure that whether you're knitting a pattern that reports a Yoke Depth or an Armhole Depth, it's at least slightly longer than your actual depth so that you have room to move your arm, and room to layer your garment if desired. Even close-fitting sweaters should have some amount of extra length added to the Armhole Depth.
This video series is accompanied by a blog post, so don't just watch the videos, go read the full post for the best learning experience!
www.kateoatesdesign.com/blog/body-measurements