The second part of their proposal concerned the way in which the subunits must be packed in the protein shell or capsid. On general grounds it was expected that subunits would be packed so as to provide each with an identical environment. This is possible only if they are packed symmetrically . A body with cubic symmetry possesses a number of axes about which it may be rotated to give a number of identical appearances. These predictions were soon confirmed and it became evident that the occurrence of icosahedral features in quite unrelated viruses was not a matter of chance selection but that icosahedral symmetry is preferred in virus structure.
Icosahedral symmetry requires definite numbers of structure units to complete a shell.
Each unit would be related identically and asymmetrically with its neighbours, and none of the units would coincide with an axis of symmetry. Within just a few years, much new and exciting information about the architecture of virus particles was acquired. Not only were the overall shapes of particles revealed but also the symmetrical arrangement of their components. This led to a need for a new terminology to describe the viral components. This terminology was generally accepted although it later proved to be inadequate.