T=13 icosahedral capsid is composed of 12 pentameric and 120 hexameric capsomeres for a total of 780 capsid proteins. These capsids are skewed, and are therefore described as either right- (dextro) or left- (laevo) handed.
D. L. Caspar, A. Klug
Cold Spring Harb. Symp. Quant. Biol. 1962; 27: 1?24