Writing a number out is only hard because English cannot agree with itself about how to do it. Three separate disagreements, all of them live, all of them switchable above.

Where the "and" goes

101      one hundred and one      British
         one hundred one          American

1005     one thousand and five    British
         one thousand five        American

1105     one thousand one hundred and five
         both, since the and belongs to the hundreds

British English puts an "and" before the last part of a number under a hundred. American English leaves it out and has done since Webster. Neither is a mistake, and the wrong one on a cheque looks careless to whoever reads it.

The rule is narrower than people think. It appears before a final part below a hundred, so "one thousand and five" but "one thousand one hundred and five", where the "and" belongs to the hundreds instead.

What a billion is

                short scale      long scale
10^9            billion          milliard
10^12           trillion         billion
10^15           quadrillion      billiard
10^18           quintillion      trillion

Britain switched officially in 1974. Most of Europe
never did.

A billion was a million million in Britain until the government switched to the American meaning in 1974. Anyone taught before then may still mean the old one, and much of continental Europe never moved at all, which is why a German Billion is a thousand times bigger than an American one.

The long scale fills the gaps with milliard, billiard and trilliard. Reading words back always takes the modern meaning, because that is what somebody typing "one billion" today intends.

Lakh and crore

12,345,678      twelve million three hundred and
                forty-five thousand six hundred
                and seventy-eight

1,23,45,678     one crore twenty-three lakh
                forty-five thousand six hundred
                and seventy-eight

Same number. The commas fall in different places
because the groups are two digits, not three.

Indian, Pakistani, Bangladeshi and Nepali English group digits in twos after the first thousand. One lakh is a hundred thousand, one crore is a hundred lakh, and the commas land in different places as a result.

This matters for anything with money in it. An amount written as ₹12,34,567 is not a typo, and a system that reformats it to ₹1,234,567 has changed the punctuation of a document without changing its meaning, which is worse than leaving it alone.

Decimals get read a digit at a time

3.14 is "three point one four", never "three point fourteen". Read the other way, 3.14 and 3.014 would both come out as "three point fourteen" and there would be no way to tell them apart.

Money is the exception, because the pence are a count of something rather than a string of digits. £1.50 is one pound and fifty pence.

Writing a cheque

Two conventions, both above. British cheques write the amount out in words and finish with "only" so nothing can be added after it. American cheques write the dollars in words and the cents as a fraction over a hundred, which is a forgery precaution: 56/100 is much harder to alter than "fifty-six".

The word "only" at the end serves the same purpose. It is why a cheque for a round amount still says "One Hundred Pounds Only" instead of stopping at Pounds.

Ordinals

Six of them are irregular and the rest take -th. The suffix trap is the teens: 11th, 12th and 13th, not 11st, 12nd and 13rd, and the rule catches everything ending in 11 to 13, so 111th and 213th too.

In words, only the last word changes. Twenty-first, one hundred and third, two thousandth.

Reading words back

The box at the bottom goes the other way. It skips anything it does not recognise and says what it skipped, so pasting a whole sentence and getting the number out of it works. Ordinals are read as their plain value, so "twenty-first" comes back as 21.

Number to Words

From words back to a number