Measurements are taken with item laid on a flat surface.
So please consider this.
If you are buying a shirt, take a like minded shirt that fits comfortably (button up, t-shirt, etc) and lay flat then measure. If your measurements fit these, perfect! I strive to make your buying experience as easy as possible!
Shoulder to shoulder: 20
Chest to Chest (underarm): 24
Tag to tail (Neckline seam above tag to bottom of shirt): 30
Sizes between brands and fabrics fit differently! Any questions please ask before purchase! Per ebay's policies, buyer pays shipping costs on returns for non fitting items, make sure to check measurements above before purchase to ensure fit!