AbstractThe pharyngeal arches are a series of bulges found on the lateral surface of the head of vertebrate embryos. In humans, and other amniotes, there are five pharyngeal arches and traditionally these have been labelled from cranial to caudal—1, 2, 3, 4 and 6. This numbering is odd—there is no ‘5’. Two reasons have been given for this. One is that during development, a ‘fifth’ arch forms transiently but is not fully realised. The second is that this numbering fits with the evolutionary history of the pharyngeal arches. Recent studies, however, have shown that neither of these justifications have basis. The traditional labelling is problematic as it causes confusion to those trying to understand the development of the pharyngeal arches. In particular, it creates difficulties in the field of congenital cardiac malformations, where it is common to find congenital cardiac lesions interpreted on the basis of persistence of the postulated arteries of the fifth arch. To resolve these problems and to take account of the recent studies that have clarified pharyngeal arch development, we propose a new terminology for the pharyngeal arches. In this revised scheme, the pharyngeal arches are to be labelled as follows—the first, most cranial, the mandibular (M), the second, the hyoid (H), the third, the carotid (C), the fourth, the aortic (A) and the last, most caudal, the pulmonary (P).