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How did Romans deal with numbers higher than 3999?

3999 is the highest Roman numeral, due to the logic that Roman numerals follow.

Roman numerals write numbers like 4, 9 and 90 in a form that reads as 'one less than five', 'one less than ten', or 'ten less than one-hundred'.

M, representing 1000, is the highest Roman numeral. Therefore, because there is no number for 5000, the number 4000 cannot follow this 'a less than b' logic. However, if one wishes to, one can write the number 4000 as MMMM.

Consequently, the Romans employed a form of notation called the vinculum to write numbers higher than 3999; or, they tended to break larger numbers down into smaller units that could be expressed numerically.

The Vinculum

The vinculum is a line written above a number, signifying a multiplication of 1000.

As such, a number such as XII with a horizontal line written above it equals 12,000.

Additionally, a box around a numeral signifies a multiplication of 100,000.

So a number such as XII with a box around it equalls 1,200,000

How did Romans deal with zero?

In Roman numerals, there is no numeral for zero. Although this is one of the greatest weaknesses of Roman numerals, and consequently one of the reasons why they have been replaced in favor of Arabic numerals, the Romans could still deal with the concept of 0.

Traditionally, zero could be written by writing the Latin word Nulla, or left as a blank space.

As the system was mainly used for trading purposes and not for abstract pure mathematics, there was little reason to devise a numeral specifically for zero.