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+# Element Call
+
+<!--- TOC -->
+
+* [Overview](#overview)
+* [Setting a custom base URL](#setting-a-custom-base-url)
+* [Why the application does not add the room path itself](#why-the-application-does-not-add-the-room-path-itself)
+* [Where the value is read](#where-the-value-is-read)
+
+<!--- END -->
+
+## Overview
+
+Element X embeds a copy of [Element Call](https://github.com/element-hq/element-call) in the
+application assets and loads it from
+`https://appassets.androidplatform.net/element-call/index.html`. Calls therefore work out of the box
+with no extra deployment, and the embedded copy is the one every user gets.
+
+Developers can point the application at another Element Call deployment instead — a local
+development server, a staging deployment, or a self-hosted one — with the **Element Call base URL**
+developer option.
+
+## Setting a custom base URL
+
+The option lives in *Settings* → *Developer options* → *Element Call*, under "Element Call base
+URL". On a release build the *Developer options* entry is hidden until the version number at the
+bottom of *Settings* is tapped seven times. Leaving the field empty restores the embedded copy.
+
+The value is used verbatim as the widget base URL, so it must be the URL of the page that joins a
+room, not the URL of the deployment root. For the public deployments that means:
+
+| Deployment | Value to enter |
+|---|---|
+| `call.element.io` | `https://call.element.io/room` |
+| `call.element.dev` | `https://call.element.dev/room` |
+
+Entering `https://call.element.io` without the `/room` suffix loads the Element Call lobby rather
+than the room, so the call is never joined.
+
+## Why the application does not add the room path itself
+
+The path that joins a room is a property of the deployment, not of Element Call. Unpacking the
+Element Call release tarball behind a reverse proxy can put it at any path — `https://call.foo.tld`,
+`https://call.foo.tld/some/path/somewhere` — and a proxy that already maps a subdomain onto
+`https://1.2.3.4/room` would end up with the suffix twice. The application passes the value through
+unchanged so that every one of those deployments can be reached.
+
+## Where the value is read
+
+`AppDeveloperSettingsView` renders the field and sends
+`AppDeveloperSettingsEvent.SetCustomElementCallBaseUrl`, which `DefaultAppPreferencesStore` persists.
+`DefaultCallWidgetProvider.getWidget` reads it back and falls back to the embedded copy when it is
+unset.