Callout numbers (aka callouts) provide a means to add annotations to lines in a verbatim block.

Each callout number used in a verbatim block must appear twice.
The first use, which goes within the verbatim block, marks the line being annotated (i.e., the target).
The second use, which goes below the verbatim block, defines the annotation text.
Multiple callout numbers may be used on a single line.

IMPORTANT: The callout number (at the target) must be placed at the end of the line.

Here's a basic example of a verbatim block that uses callouts:

[source,subs=-callouts]
....
include::ex-callout.adoc[tag=basic]
....

Here's how this example gets rendered:

====
include::ex-callout.adoc[tag=basic]
====

Since callout number can interfere with the syntax of the code they are annotating, Asciidoctor provides several features to hide the callout numbers from both the source and the converted document.
The sections that follow detail these features.
