What do you call numbers such as $100, 200, 500, 1000, 10000, 50000$ as opposed to $370, 14, 4500, 59000$

There are different categories of numbers that we use every day.

  1. Integers that written in decimal notation have $1, 2$ or $5$ as the leading figure, followed by none, one or more zeros. These are very common numbers, e.g. used in Bank notes: $1, 2, 5, 10, 50, 200, 1000, ....$

  2. Other intergers, which are less common, e.g. your (approximated) height in $cm$, the (approximate) temperature of your body or the environment, or the (approximate) result of converting $200$ miles to kilometers.

  3. Numbers that are not integers, such as $\$1.23$.

If you have $\$198$ (category $2$) bill in a restaurant, you'd probably tip $\$2$ to round it to $\$200$ (category $1$).

Is there a term for numbers of category $1$?


The first category is known as the 1-2-5 series, and it is an example of a system of preferred numbers. As the name implies, there is nothing mathematically distinctive about such numbers; humans just prefer them.


The OEIS calls your first set a "Hyperinflation sequence for banknotes", though there have been many coins and banknotes with different denominations around the world, such as a $1935$ Canadian $\$25$ note.

Mathematically they are the numbers generated by $$\frac{1+2 x+5 x^2}{1-10 x^3}.$$

People have counted with other patterns, notably the Babylonian sexagesimal system which we still use for minutes and seconds and so often think of $15$ and $30$ as round in some contexts.