What does MCMXCI mean?

MCMXCI is the Roman numeral for 1,991 — 1000 + 900 + 90 + 1.

How do you write 1991 in Roman numerals?

Take the largest value that fits and repeat until nothing is left: M (1000), then CM (900), then XC (90), then I (1). That gives MCMXCI.

1,991 in Roman numerals

MCMXCI

Number (1–3,999 for numerals)

Roman numeral

1,991 is written MCMXCI in Roman numerals. That is 4 symbol groups added together, reading left to right from the largest value down.

How it breaks down

MCMXCI = 1000 + 900 + 90 + 1 = 1,991
Group	Value	Subtraction	Running total
M	1,000	—	1,000
CM	900	M − C	1,900
XC	90	C − X	1,990
I	1	—	1,991

Why the smaller symbol comes first

MCMXCI uses subtractive notation in 2 places: CM means M minus C, or 900; XC means C minus X, or 90. A smaller symbol placed before a larger one is subtracted from it rather than added, which is why IX is nine and not eleven.

Only six such pairs are valid — IV, IX, XL, XC, CD and CM. That restriction is what makes the notation unambiguous: IIII and IC both look like they should work, but neither is standard, and only one spelling of any number is correct.

The symbols

Symbol	Value	Used here
I	1	yes — 1×
V	5	no
X	10	yes — 1×
L	50	no
C	100	yes — 2×
D	500	no
M	1,000	yes — 2×

The same number, other ways

In words: One thousand nine hundred ninety-one
Roman numeral: MCMXCI
Symbol count: 6 characters
Binary: 11111000111
Hexadecimal: 7C7

The numbers either side

Number	Roman numeral
1,989	MCMLXXXIX
1,990	MCMXC
1,991	MCMXCI ←
1,992	MCMXCII
1,993	MCMXCIII

Common questions

What is 1991 in Roman numerals?: 1,991 is MCMXCI.
What does MCMXCI mean?: MCMXCI is the Roman numeral for 1,991 — 1000 + 900 + 90 + 1.
How do you write 1991 in Roman numerals?: Take the largest value that fits and repeat until nothing is left: M (1000), then CM (900), then XC (90), then I (1). That gives MCMXCI.

Sources

Calculated on this page — At One Place

How these figures are compiled and checked