The number is written two ways because, even if one is incorrect, the identical error cannot occur in the other. It's half a joke about the legal thing, half an emphasis on the number (when it's extremely significant).
In English, we often write numbers with letters instead of digits. These are called alphabetical symbols or alphabetic characters. The five most common alphabetical symbols are listed below with their corresponding numbers: '1' stands for zero, '2' for one, and so on up to '9'. There are also several letters used in mathematics and science that do not represent actual numbers; they are called mathematical symbols.
People wrote numbers first as words, then as symbols. You can still see this in action today when counting money. Or rather, counting bills! The person doing the counting will say something like "one hundred dollars", and the others will repeat the amount they think was counted. This process ensures that nobody gets cheated. The original purpose of counting money was to avoid theft by having a third party verify that the right amount was given to each person. But now that we use computers for this task, it's possible to write software that does the same thing automatically without any risk of fraud.
During the Renaissance, Italians started using '0' as a digit. Before this time, numbers were written out completely with no omissions.
The benefits of simply writing numbers are self-evident. There is just one number that has to be put or altered. It also saves space and makes a document appear less "legal." Furthermore, putting down the number in words as well as digits reduces the likelihood of making a mistake. Finally, this practice provides some protection against fraud; if someone tries to pass off another person's work as their own, they are more likely to make a mistake when writing a number.
When writing out numbers it is important to follow certain conventions to ensure accuracy. Numbers should be written in full, with no abbreviations. Abbreviations are commonly used for dates (e.g., 4/4/04), but these cannot be used for numbers. Instead, numbers should be written out in whole years (e.g., 2004), in whole months (e.g., April), or in whole days (e.g., Wednesday). Years, months, and days can also be expressed as fractions (e.g., 1/12) or integers (e.g., 12). Fractions must be written in numerator-denominator form, with no symbols such as dots or slashes used in place of the denominator. For example, 3/4 means three quarters or thirds.
Numbers within texts require special attention because writers often make mistakes when typing or copying text from other sources.
A fundamental guideline for writing numbers is that little numbers from one to ten (or one to nine, depending on the style guide) should be spelt explicitly. Larger numbers (those more than 10) are written as numerals.
In English language newspapers and magazines, it is customary to spell out numbers from 7 to 9 even if they are single digits. This is because readers of such publications are likely to know how many letters are in a word, so they can work out the number they are reading about. For example, an article titled "7 ways to kill your writer's block" would not be understood unless you read the whole thing!
Numbers less than seven are usually not spelled out, although some publications may do so as a matter of custom. For example, a newspaper might spell out numbers between 1 and 6, but not 0 without making this clear from the context.
As well as being polite and respecting others' time, spelling out numbers also helps people who have difficulty with mathematics or reading understand what level they are reading at. A mathematician or scientist reading about research studies using calculus could tell immediately that these are large numbers because they need to be written out in full.
Finally, spelling numbers out makes them easier to find in old articles or books.