Look at a clock face, a film credit at the end of a movie, or the chapter numbers in a book. You'll often spot letters like I, V, X, L, C standing in place of numbers. That's the Roman numeral system — the way the ancient Romans wrote numbers more than 2 000 years ago. In Year 4 we learn to read and write them up to 100.
Why we still use them
Roman numerals stuck around for decoration and order. You'll find them on:
clock and watch faces (IV, IX, XII for 4, 9, 12),
the front matter of books (Chapter II, III, IV …),
film and game sequels (Star Wars VII),
monuments and gravestones (MCMXLV = 1945),
monarchs' names (Queen Elizabeth II, Henry VIII).
For everyday arithmetic we use the Hindu-Arabic numerals (0–9) — Roman numerals are slow to add and have no zero. But they are still part of culture.
The seven symbols
Roman numerals use seven capital letters as symbols.
Letter
Value
I
1
V
5
X
10
L
50
C
100
D
500
M
1 000
In Year 4 we only need the first five — I, V, X, L, C — because we work up to 100. The letters D and M will come in later years for bigger numbers.
A handy memory trick: I V X L C D M — "I Value X-rays Like Cats Drinking Milk." Silly, but it sticks.
Reading them — the basic idea
Each letter has a value, and you usually add them up.
III = 1 + 1 + 1 = 3
VI = 5 + 1 = 6
XV = 10 + 5 = 15
LXX = 50 + 10 + 10 = 70
CL = 100 + 50 = 150
The letters are written from biggest to smallest, left to right. When that's the case, you just add.
The subtraction trick
There's one twist. When a smaller letter comes before a bigger one, you subtract instead of adding.
IV = 5 − 1 = 4
IX = 10 − 1 = 9
XL = 50 − 10 = 40
XC = 100 − 10 = 90
Without the subtraction trick, the Romans would have written 4 as IIII (1+1+1+1). Their shortened version IV is cleaner — and you'll often see it on watches (although some watch faces use IIII anyway, just for symmetry).
Only four subtractive pairs appear up to 100: IV, IX, XL, XC. There are more for bigger numbers (CD, CM), but they don't appear in Year-4 work.
