Diagram illustrating an ordination of the serological differentiation indices (SDI’s) in Table 2 of van Regenmortel (1975); the SDI between HRV and CGMMV was taken as 6.5 (M. H. V. van Regenmortel, pers. comm.). The first dimension contains 43% of the ‘information’ in the ordination, the second 28% and the third 18%; altogether 89%.