The second rule - How many times in a row may the basic symbols be repeated in a Roman numeral

Numerals repetition - Repetition of certain symbols in a number:

• These symbols: I (1), X (10), C (100), M (1,000), (X) (10,000), (C) (100,000) and (M) (1,000,000) may occur no more than three times in a row in any number.

• These symbols: V (5), L (50), D (500), (V) (5,000), (L) (50,000), (D) (500,000) may occur only once in any number.

Examples:

• Number 4 is written as a numeral as: IV and not IIII (though the IIII form circulated as an additive form);
• Number 5 is written as a numeral as: V and not IIIII;
• Number 10 will be written as: X and not VV, nor VIIIII;
• Number 100 will be written as: C and not LL, nor LXXXXX, nor XXXXXXXXXX.