Decoding Roman Numerals

The date 1750 in Roman numerals (floor of Santa Maria Maggiore)

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Roman numerals, which first started to appear between 900 and 800 BCE, are made up of seven basic symbols: I, V, X, L, C, D, M (1, 5, 10, 50, 100, 500, 1000). 

In forming numbers there are three basic rules:
1. When a symbol appears after a larger (or equal) symbol it is added: VI (6).
2. If the symbol appears before a larger symbol it is subtracted: IV (4). 
3. Don't use the same symbol more than three times in a row. For example, 40 is XL not XXXX.  

Alternative, but much less common, symbols for 1000 and 500 were, respectively, CIƆ and IƆ. Centuries later, the two symbols make an occasional reappearance, as we can see, for example, in Santa Maria Maggiore, where a date (1750) inscribed on the floor of the church is represented as CIƆIƆCCL rather than MDXXL. Another example appears in the inscription above the Fontana della Botticella:  CIƆIƆCCLXXIIII rather than MDCCLXXIIII (1774).

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Fontana della Botticella (Fountain of the Barrel)