# System expansion: Introduction
> Johnny.Decimal is simple by design. But life doesn't always fit in a neat box. Here are three options for system expansion.
## The problem
The Johnny.Decimal structure is two-dimensional by design – areas and categories. This limitation is part of what makes it powerful. It's hard to get lost in a simple space.
But sometimes an extra dimension is required. Maybe you need to make more room in one part of a standard system. Or perhaps you have two or more systems and need to differentiate between them.
This page provides an overview of the three options. More detail is provided on the following pages.
## Definitions
**The Johnny.Decimal system** - A collection of methods, tools, and behaviours as documented at johnnydecimal.com.
**Your Johnny.Decimal system** - Your implementation of this system, customised to suit whatever it is that you're organising.
**Components** - Johnny.Decimal systems are made up of components. The list of components is not restricted, but the following are common: the JDex, filesystem, notes, tasks, to-dos, reminders, email, calendar, bookmarks.
**Domains** - Systems with one or more shared components are grouped by domain. For example, when you go to work and everything is on the company's systems, 'work' is its own domain. It is completely separate from your home system, your 'personal' domain.
**AC.ID notation** - Shorthand to refer to any generic Johnny.Decimal system. It stands for [area, category, ID](/documentation/acid-notation/).
## Multiple systems
Consider this option if you need to differentiate between two or more Johnny.Decimal systems.
### Example: Home and work
You have a system at home to manage your personal life. When you go to work, the company has its own system. So you have two separate systems and you need to differentiate between them.
#### Solution
Add a unique identifier to the start of each system.
For more information, see multiple systems [Overview](/documentation/multiple-systems-overview/) and [Guidelines](/documentation/multiple-systems-guidelines/).
## Expand an area
Consider this option if most of your life fits neatly in to a standard Johnny.Decimal system. But you have one area that doesn't. Either it has more than 10 of something, or it requires more depth than the system allows.
### Example: Students or freelancers
You're a student with more than 10 classes. Or you're a freelance designer with more than 10 clients, each with more than 10 products that you work on.
#### Solution
Expand one of your areas to accommodate.
For more information, see expand an area [Overview](/documentation/expand-an-area-overview/) and [Guidelines](/documentation/expand-an-area-guidelines/).
## Extend the end
Consider this option if almost everything in your system fits neatly in to the standard Johnny.Decimal structure. But you have an ID that needs to be repeated, or you require more than 100 IDs in a category.
### Example: Chickens
You're using the standard [Life Admin System](/las/) and have many chickens. You need a way to organise your visits to the vet.
#### Solution
Extend the end to cover each chicken. `11.63+ Wendy` is the ID for Wendy's visits to the vet.[^the-plus]
[^the-plus]: Don't worry about the `+` for now, we'll talk about it later.
### Example: A repeated task within a larger project
In my old job, `72.02` was the task of installing cabling at a data centre. But there were 13 data centres.
#### Solution
Extend the end to cover each data centre. `72.02+ X01` was the centre with site code `X01`, `72.02+ Y02` had site code `Y02`, etc. This worked because most of the other IDs in this system didn't need to occur 13 times.
For more information, see extend the end [Overview](/documentation/extend-the-end-overview/) and [Guidelines](/documentation/extend-the-end-guidelines/).