# AC.ID notation

> Johnny.Decimal numbers are made up of the area-category combo before the decimal, then the ID after. We call this AC.ID notation.

As we have seen, the number before the decimal tells us what area and category we're in. The number after the decimal is the ID.

I often use AC.ID notation as shorthand to refer to any generic Johnny.Decimal system. `A`, `C`, and `ID` serve as variables.

For example:

- `AC.ID` is any ID in the system.

- `1C.ID` is any ID in area `10-19`.

- `11.ID` is any ID in category `11`.

- `AC.11` is ID `.11` in any category.

See this in action on the first two figures of the [standard zeros](/documentation/the-standard-zeros/) page.

## SYS.AC.ID

When we use [multiple systems](/documentation/multiple-systems-overview/) the notation is extended to `SYS.AC.ID` – System Area Category ID.