Rome counted with additive letters until place value won out
Roman numerals dominated European number writing into the Late Middle Ages, summing symbols like C for 100. From the 1400s, Hindu–Arabic place value gradually replaced them, yet years on monuments, clocks, and film copyrights keep the old alphabet alive. Modern style uses subtractive IV and IX rather than IIII and VIIII.
Seven Latin letters do the work: I, V, X, L, C, D and M stand for 1, 5, 10, 50, 100, 500 and 1000, and XXVII adds up to 27. There is no zero and a separate symbol for each power of ten, so empty places are simply left out, giving 1776 as MDCCLXXVI, the date on the Statue of Liberty's tablet. Only six subtractive pairs are standard: IV, IX, XL, XC, CD and CM. The modern system tops out at 3999, and before M existed the Romans could write only up to 899.
Practice was never standardised. Additive forms like XXIIII for 24 persisted beside subtractive ones, and the Colosseum's gates mix the two, labelling gate 44 XLIIII. Clock faces still often show IIII at four but IX at nine, a habit going back to the late-fourteenth-century Wells Cathedral clock, though the Elizabeth Tower clock at Westminster uses IV. Soldiers of the eighteenth legion wrote their number XIIX, a form that may echo Latin duodeviginti, two from twenty, and Admiralty Arch in London gives 1910 as MDCCCCX.
Zero was expressed with the word nulla, and N for none survived in apothecaries' prescriptions into the twentieth century. Fractions were duodecimal, since twelve divides neatly by three and four, with S marking a half, as in VIIS for seven and a half. For thousands the Romans borrowed an Etruscan device, the apostrophus, writing 1,000 as CIↃ and 10,000 as CCIↃↃ; that CIↃ probably shaped the later M, and John Wallis may have modelled his infinity sign on its variant ↀ.
Source: Roman numerals
