# Substitutions

Substitutions are variables. They are declared in any document and defined in the `conf.py` file, `rst_prolog` setting.

#### CAUTION
Do not use substitutions in headings. The reason is the text that replaces the variable may be longer than the line that is over or below the text and this will produce an error.

## Default substitutions

Projects using the theme can use the following substitutions:

* `|v|` for <i class="inline-icon icon-check" aria-hidden="true"></i>
* `|x|` for <i class="inline-icon icon-cancel" aria-hidden="true"></i>

## Substitutions within code blocks

To add substitutions within [Code blocks](https://sphinx-theme.scylladb.com/stable/examples/code-blocks.md), pass the option `:substitutions:` to the `code-block` directive.

For example:

```rst
.. code-block::
  :substitutions:

  |rst|
```

Renders as:

```default
restructuredText
```