### Top-level application { #top-level-application }

First, create the main, top-level, **FastAPI** application, and its *path operations*:

{* ../../docs_src/sub_applications/tutorial001.py hl[3, 6:8] *}

### Sub-application { #sub-application }

Then, create your sub-application, and its *path operations*.

This sub-application is just another standard FastAPI application, but this is the one that will be "mounted":

        message[32] = 112; // Offset
        message[33] = 0;
        message[34] = 0;
        message[35] = 0;

        // User (8 bytes "user" in Unicode at offset 124)
        message[36] = 8; // Length
        message[37] = 0;
        message[38] = 8; // Max Length
        message[39] = 0;
        message[40] = 124; // Offset
        message[41] = 0;
        message[42] = 0;

So we put them in their own `dependencies` module (`app/dependencies.py`).

We will now use a simple dependency to read a custom `X-Token` header:

//// tab | Python 3.9+

```Python hl_lines="3  6-8" title="app/dependencies.py"
{!> ../../docs_src/bigger_applications/app_an_py39/dependencies.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="1  5-7" title="app/dependencies.py"