Bliss is essentially an alphabetical classification, and uses a combination of letters and numbers to order books by subject. So ‘I’ appears before ‘IA’, which in turn precedes ‘IB’, and so on. In this way IC is shelve before ICA, ICB etc. For example:
63 M = European history
63 MV = European history –Britain
63 MVO = European history –Britain– 1640-1660