# Multiple systems: Overview

> To manage multiple Johnny.Decimal systems, assign an identifier to the front of each one. We refer to this as SYS.AC.ID.

## The problem

If you manage two or more Johnny.Decimal systems, you need a way to identify them. For example, you have a system at home and at work.

The two systems have nothing in common. But they both contain the ID `11.11`. When you use this ID, to which system are you referring? It is now ambiguous.

## The solution

We recognise that these two (or more) systems are distinct and separate, and assign each of them a 'system identifier' before `AC.ID`.

Use three characters of the form `[A-Z][0-9][0-9]`, i.e. `A00 … Z99`. We refer to this as `SYS.AC.ID`.

### Example

We assign our home system the identifier `H01` and our work system `W01`. The full ID for `11.11` in each of these systems is now `H01.11.11` and `W01.11.11`, respectively.

I encourage you to be more creative with identifiers. Interesting letter/number combinations will help your brain.

## FAQs

### What am I extending/gaining?

This scheme extends the number of available systems from one (the classic `AC.ID`) to 2,600.

### How does this affect my IDs?

Within each system, IDs are unaffected. Each system is a standard implementation of Johnny.Decimal.

### How does this affect my JDex?

If your JDex contains entries for both systems, each entry must include the full `SYS.AC.ID` identifier. Otherwise, searching for `11.11` might return both results, which is ambiguous.

If your systems exist in separate domains, you may not need to do this. See the [Guidelines](/documentation/multiple-systems-guidelines/).

### Can I use this with expand an area?

Yes. You may expand an area of a `SYS.AC.ID` system.

### Can I use this with extend the end?

Yes. You can extend the end of a `SYS.AC.ID` system.

### When should I use/not use this?

Use this when you have two or more systems in the same domain. Or you may choose to label systems in separate domains for clarity – like in our home/work example.

Avoid creating multiple systems where possible. Always prefer to 'fill up' an existing system. More systems equals more complexity. Complexity is to be avoided.

Most people can fit their entire personal life in one Johnny.Decimal system. If you've run out of room, try to compress your system before creating another.