Icosahedral capsids with a triangulation number T=7 are composed of 12 pentameric and 60 hexameric capsomeres for a total of 420 capsid proteins. These capsids are skewed, and are therefore described as either right- (dextro) or left- (laevo) handed.
Papillomaviruses and Polyomaviruses are composed solely of pentameric capsomeres for a total of 360 capsid proteins. Nevertheless their icosahedral structure is that of a T=7 capsid in which pentamers occupy hexavalent positions.
Icosahedric triangulation number is calculated following Caspar and Klug system
D. L. Caspar, A. Klug
Cold Spring Harb. Symp. Quant. Biol. 1962; 27: 1?24