2470: Codex & Affix: Why Different Plurals? Sep 16, 2021

The plural of 'index' is 'indices' and likewise with 'codex' to 'codices', but the plural of 'affix' is 'affixes'. This is not in fact inconsistent, but a result of the way in which English borrows words from Latin. In the case of 'codex', this is a Latin word in the 3rd declension which had a Latin plural of 'codices'. Strictly speaking, the plural is simply '-ēs' but the 'x' is assimilated along with the vowel as seen in the rest of the forms including the genitive singular 'cōdicis'. In the case of 'affix', this is formed from the prefix 'ad-' (toward) and 'fixus', which is the participial form of the verb 'figere' meaning 'to fasten', as in 'join together'. Since English generally does not take the 2nd declension '-us-' ending, 'affixus' (the -d- was assimilated) becomes mere 'affix'.