Neck rings are used to enable the head to turn more easily on the body. We have a video showing their use here (at the bottom of the page):
http://pictures.bountifulbaby.com/videos/videos.html.
The diameter of the neck ring is measured from the widest point. If referencing the picture that would be the bottom part.
Click this link for IMPORTANT additional information about the plugs and rings.Click here to open more information and/or photos for this item.
