The posterior glottis is an area of the larynx previously referred to by the terms 'posterior commissure' and 'interarytenoid'; these are poorly defined and a new definition of this unique area of the larynx is provided. Within the text is a series of experiments performed on nearly 300 larynges. The posterior glottis was examined in relation to the following: the embryology, the epithelium, mathematical dimensions, gross anatomy, microanatomical structures, submucosal spaces and the spread of carcinoma related to this area. Various significant findings were made. There exists a pharyngoglottic duct which divides the embryonic larynx into anterior (membranous) and posterior (cartilaginous) parts. The epithelium of the posterior glottis in neonates and non-smokers is respiratory in nature and the notion of a laryngeal respiratory function is reinforced by a mathematical analysis of the cross sectional areas of the larynx during inspiration. The presence of a posterior cricoarytenoid ligament which stabilizes the arytenoid is confirmed. The spread of carcinoma to the posterior glottis from the different primary laryngeal and pyriform fossa sites shows differing modes of invasion but in particular a direct extension and connection with the subglottis. The most significant conclusion from these studies is that the posterior glottis is so intimately related to the subglottis that it must be considered as part of the subglottis. The acceptance of this fact, however, requires a new definition of the larynx from that given by the American Joint Committee on Cancer Staging and that of U.I.C.C. The posterior glottis considered as part of the subglottis requires a review of our present understanding of the structure and function of the larynx and in particular the spread of cancer posteriorly.