What does CMXXI mean?

CMXXI is the Roman numeral for 921 — 900 + 10 + 10 + 1.

How do you write 921 in Roman numerals?

Take the largest value that fits and repeat until nothing is left: CM (900), then X (10), then X (10), then I (1). That gives CMXXI.

921 in Roman numerals

CMXXI

Number (1–3,999 for numerals)

Roman numeral

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

How it breaks down

CMXXI = 900 + 10 + 10 + 1 = 921
Group	Value	Subtraction	Running total
CM	900	M − C	900
X	10	—	910
X	10	—	920
I	1	—	921

Why the smaller symbol comes first

CMXXI uses subtractive notation in one place: CM means M minus C, or 900. 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 — 2×
L	50	no
C	100	yes — 1×
D	500	no
M	1,000	yes — 1×

The same number, other ways

In words: Nine hundred twenty-one
Roman numeral: CMXXI
Symbol count: 5 characters
Binary: 1110011001
Hexadecimal: 399

The numbers either side

Number	Roman numeral
919	CMXIX
920	CMXX
921	CMXXI ←
922	CMXXII
923	CMXXIII

Common questions

What is 921 in Roman numerals?: 921 is CMXXI.
What does CMXXI mean?: CMXXI is the Roman numeral for 921 — 900 + 10 + 10 + 1.
How do you write 921 in Roman numerals?: Take the largest value that fits and repeat until nothing is left: CM (900), then X (10), then X (10), then I (1). That gives CMXXI.

Sources

Calculated on this page — At One Place

How these figures are compiled and checked