The numeric system represented by Roman numerals originated in ancient Rome and remained the most common way of using numbers throughout Europe until the Late Middle Ages.
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Roman numerals are represented by combinations of letters from the Latin alphabet and are based on seven symbols:
Symbol I V X L C D M
Value 1 5 10 50 100 500 1,000
The use of Roman numerals continued even after the decline of the Roman Empire. However, from the 14th century on, they began to be replaced by Hindu-Arabic numerals; despite this, the use of Roman numerals persists in some minor applications.
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How to read Roman numerals
In Roman numerals, letters are used to indicate certain “base” numbers, and arbitrary numbers are indicated using combinations of symbols, through an additive and subtractive system.
The original model for Roman numerals used the symbols I, V, and X (1, 5, and 10) as simple counting marks. Each marker for 1 (I) added a unit value up to 5 (V), and was then added to (V) to create the numbers 6 to 9:
I, II, III, IIII, V, VI, VII, VIII, VIIII, X.
The numbers for 4 (IIII) and 9 (VIIII) proved problematic as well as being easily confused with III and VIII, therefore they were replaced with IV (one less than 5) and IX (one less than 10). This feature is called subtractive notation.
Since the Roman numeral system is decimal, tens and hundreds follow the same pattern.
So from 10 to 100 (counting in tens, with X in place of I, L in place of V and C in place of X):
X, XX, XXX, XL, L, LX, LXX, LXXX, XC, C.
Note that 40 (XL) and 90 (XC) follow the same subtractive pattern as 4 and 9.
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Why have they been used for a long time?
The direct influence of Rome over a long period, the superiority of its numbering system, and the strength of tradition explain the strong position the system held for nearly 2,000 years in the trade, in scientific and theological literature, and belles lettres. Its advantage was due only to the memorize the values of only four letters V, X, L, and C, and it was easier to add numbers, the most basic arithmetic operation.
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Although the changes in these numbers in their forms from the 3rd-century bce are well known, the origin is obscure. The theory of the German historian Theodor Mommsen (1850) was widely accepted as it was argued that the V for five represented the open hand. Two of these gave X times 10, and the L, C, and M were modifications of Greek letters. Furthermore, following the study of the inscriptions left by the Etruscans, it shows that the Romans adopted the Etruscan number system starting from the 5th-century bce, but while the Etruscans read their numbers from right to left, the Romans read them from left to right. L and D for 50 and 500, emerged in the Late Roman Republic, and M did not come to mean 1,000 until the Middle Ages.
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Are they still being used today?
Some uses that are rare or never seen in English-speaking countries are relatively common in continental Europe. For instance: Capital or small capital Roman numerals are widely used in Romance languages to denote centuries, e.g. the French xviiie siècle and the Spanish siglo XVIII mean “18th century”
Mixed Roman and Hindu-Arabic numerals are sometimes used in numeric representations of dates (especially in formal letters and official documents, but also on tombstones). The month is written in Roman numerals, while the day is in Hindu-Arabic numerals: “14.VI.1789” and “VI.14.1789” both refer unambiguously to 14 June 1789.
In Italy, where roads outside built-up areas have kilometer signs, major roads and motorways also mark 100-meter subdivisions, using Roman numerals from I to IX for the smaller intervals. The sign “IX | 17” thus marks kilometer 17.9.
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https://en.wikipedia.org/wiki/Roman_numerals http://mathworld.wolfram.com/RomanNumerals.html https://www.britannica.com/science/numeral