Use the /inc directory to declare any theme functionality. All files in this directory are imported inside of functions.php.

The /inc directory is organized into sub-directories based on the functionality/purpose of the code. These directories can be modified as needed, but the following structure is recommended:

inc/
└─── customizer/    (functions relating to the theme customizer)
└─── functions/     (general functions that don't fit into any other directory)
└─── hooks/         (theme hooks)
└─── post-types/    (theme post type registrations & functions)
└─── setup/         (functions relating to the theme setup)
└─── shortcodes/    (shortcode registrations)
└─── template-tags/ (functions that render markup for use in theme templates)
└─── README.md

As a general rule, each .php file should contain a single function/action which should match the name of the file (replacing underscores with hyphens in the filename).

For example, function demo_function() {...} would be declared in a file named demo-function.php and stored inside an appropriate /inc sub-directory.