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The preposition per was originally Latin - so in an old-fashioned view should be italicised, per. In the view of AWE it has been so thoroughly naturalised in English that italics are no longer needed; however, pedants (including AWE) prefer phrases involving the preposition + a Latin noun to be italicised together, like per annum (each year). (If you prefer to write in English per year, it need not be italicised.) In English, the normal meaning of per is 'for each one' [of the noun]; but this meaning only developed long after the Romans, in the 15th or 16th centuries.

In Latin, prepositions control the case of the noun to which they relate. In the case of per, that case was the accusative. So the Latin word for 'year', annus (in the nominative), becomes per annum, and the full form of the abbreviation per cent. is per centum.
Avoid the typographical (or spelling) error of writing annum with a single '-n-'. The wealthy father of a boy at a British public school received a bill for fees 'per anum', and replied that he preferred to pay "in the usual way - through the nose" - realizing that the Latin he had just read meant 'through the anus'.
One particular quirk of correctness arises from the case of per cap., which means 'per head'. The Latin for 'head' is caput (for both nominative and accusative cases), so the strict translation of 'per head' should be per caput. Yet it is usually given as per capita, which is the plural: 'per heads'. This seems illogical to those who know Latin. Fowler (1926) says that it is "a modern blunder" and "something of a barbarism"; Burchfield says "but attitudes have now changed and [per capita] is now in standard use beside per caput". Student writers should feel free to use either - unless they know that their teachers care about such pedantry. AWE finds per caput to be rare nowadays, but The Economist style guide has ruled (and explained): "Per caput is the Latin for per head. Per capita is the Latin for by heads; it is a term used by lawyers when distributing an inheritance among individuals, rather than among families (per stirpes). Unless the context demands this technical expression, never use either per capita or per caput but per person."