# Important terms & definitions

A bit is normally structured like this:

```bitmark
[.bitType]
[@id:123]

[%item][%lead]

[!instruction]

Body
```

It consists of a **bit type**, an **ID**, an **item**, a **lead**, an
**instruction**, the **body** (the actual content) and, optionally,
**resources**.

## Bit type

- Defines the type of the bit, e.g. `[.article]` or `[.image]`.
- There are various bit types for various kinds of content — the content
  defines which bit type to use (text → `.article`, an image → `.image`, …).
- The text of a bit is formatted by default; only plain text is declared,
  as in `[.article:text]` (see [Text formatting](https://docs.bitmark-association.org/concepts/text-formatting.md)).

## ID

The ID (`[@id:…]`) is a unique reference number given to each bit. It
identifies the bit and is crucial for referencing, functions and rights
management. **Never change the ID of a bit.**

## Item

- Information identifying or referring to a specific bit, like the number
  of a provision, a figure or an exercise — it answers the question "which bit?".
- A bit has to work without its item too; it is a kind of metadata.
- Examples: `1.`, `1a`, `Art.2`, `Number 3`, `A)`, `Page 27`.

```bitmark
[.bitType]
[%item][%lead]
Body
```

## Lead

- Information ("label") classifying or describing the content of a bit,
  like "Example" or "Note".
- The lead is not the title, and a bit must work without it; if there is
  only one lead in a whole book, it is probably not a lead.
- Examples: `Example`, `Note`, `Case`, `Side Note`.

## Instruction

- The instruction (`[!…]`) addresses the user; it can also serve as the
  title of a bit (not of the content — headings are bits of their own, see
  [Headings and paragraphs](https://docs.bitmark-association.org/concepts/headings-and-paragraphs.md)).
- Examples: "Fill out the gaps", "Answer the question", "Read the text".

## Body

The main element of the bit — normally text:

```bitmark
[.bitType]

[!instruction]

This is the body of the bit.
```

## Resources

Bits often contain resources: pictures, audio, video, documents, … Every
bit can have one resource attached by default. For the resource to be
recognized it must be declared:

- In the bit type, combined with `&` — e.g. `[.article&image]`,
  `[.cloze&audio]`.
- In the body, as a `&` tag with the link to the resource.

```bitmark
[.article&image]

[!instruction]

[&image:https://example.org/images/background.png][@caption:Caption Text]

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
```

Placement:

- A `&` resource is displayed where the app that shows the bit puts it.
- Inside formatted text, an image can also be placed inline, at an exact
  position in the text (see [Text formatting](https://docs.bitmark-association.org/concepts/text-formatting.md)).