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There are 24 hours in one day, but the day can be measured by splitting it into two halves. The first 12 hours of the day - from midnight to midday - are called AM, and the next twelve hours are called PM.

The Roman numeral XII, meaning 12, next to a brain
Image caption,
X is 10, so XII is 10 + 2, or 12

Roman numerals

The Romans used a different numbering system from the one that we use today. Their numbers were shown by letters.

I = 1, V = 5, X = 10, L = 50 and C = 100

Here are some examples

V is 5, so VI is 5 + 1 or 6.

IV is 4. When the I is before the V it means 1 less than 5.

The Roman numeral XII, meaning 12, next to a brain
Image caption,
X is 10, so XII is 10 + 2, or 12