Measurement A is your preferred neck opening width. If you have a garment with a neck width you like, lay it flat and measure the neck opening between the shoulder seams. If you’re measuring someone, take care your measuring is done without curving your measuring tape around the neck. The actual measure will go through the column of the neck, with no bend around the curve of the back or front of the neck.
Measurement B is shoulder width. It’s important this measure is made at the correct spots without curving around the column of the neck.
To find the correct point to measure (Point B), press the tip of your finger into the shoulder at the yellow arrow location shown in the image. You should feel the valley created by the joining of the arm and shoulder. When the arm is raised you should feel the shoulder bones against one side of your finger and the top of the upper arm bone rotating against the other. Mark this spot (Point B) at the very top of the shoulder with a small piece of tape or marker dot. This location, Point B, is used for four measurements. Find the same point on the other shoulder (this one does not need to be marked as it will be used only once). Measure between Point B and this spot for the shoulder width measurement (Measurement B).