645 in words

645 is written "six hundred forty-five". British English inserts "and" before the final part, giving "six hundred and forty-five"; American English leaves it out.

Where each word comes from

The same number, other ways

In words (US)

Six hundred forty-five

In words (UK)

Six hundred and forty-five

Ordinal

645th — six hundred forty-fifth

Roman numeral

DCXLV

Digits

645

Writing it on a cheque

On a cheque the amount line reads "Six hundred and forty-five and 00/100" — or "Six hundred and forty-five only" in Indian and much of Commonwealth practice. The fraction or the word "only" exists to stop anyone appending digits after what you wrote. Draw a line through any space left over for the same reason.