Decoding Roman Numerals

The Year 1750 in Roman Numerals (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, eg. VI.
2. If the symbol appears before a larger symbol it is subtracted, eg. IX. 
3. Don't use the same symbol more than three times in a row. For example, 40 is XL not XXXX.  

The figure 1,000 was originally represented by the Greek letter phi Φ, which, in time, morphed into CIƆ.  Half of Φ, in other words IƆ, was adopted for 500.  

Centuries later, the two symbols make an occasional appearance, as we can see, for example, in Santa Maria Maggiore, where the year 1750 is inscribed on the floor of the church 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)