The Neck measurement is used in fitting any garment or accessory meant to sit closely around, or upon, the neck: collared shirts, gorgets, doublets, chokers, and early ruffs, to name a few.
The pattern making, the Neck measurement is taken at the base of the neck, with the tape situated at the join between the neck and the body. This is not, generally, level nor quite what you’re used to taking to measure for buying men’s shirts. The tape should appear to dip slightly in front as you follow the line of the neck. You must be looking straight ahead, with your weight evenly distributed on both feet or sitting up straight with both feet on the floor to take this measurement accurately.
The neck, like many areas of a fully developed human body, is a conic, rather than cylindrical, solid. In other words, it’s not straight upsy-downsy – it wings out a bit at the sides for the shoulder muscles, and dips a touch at the front just to make patterning more difficult. (Making it more of an irregular truncated oblique conic solid, if you want to get really technical.) If you take the measurement midway up your neck, with the tape completely level, you will get a smaller measurement. If you use that smaller measurement as the basis for, say, a standing collar on a doublet, you will end up with a doublet that is very tight at the base of the neck and uncomfortable to impossible to wear (and a new sort of sympathy for dogs on choke-collars).
You can generally take this measurement yourself, if you can remember not to tense your shoulders up. If you’ve got very developed muscles in your shoulders (due to athletics or years of back-pack-with-40lbs-of-text-books), and plan to make a fully fitted tall collar, you will want to take two neck measurements: one at the bottom of your neck, and one at the middle. You may also want to take a Front and Back neck measurement, as the majority of your neck falls to the front. In this case, front and back are divided at the top of the shoulder, where a shoulder seam would sit so that it is seen on neither the front nor the back of the body when viewed straight on.