Middleware runs according to path not router

_includes/api/en/4x/router-use.md

@@ -77,3 +77,26 @@ app.use(express.static(__dirname + '/uploads'));
 
 The `router.use()` method also supports named parameters so that your mount points
 for other routers can benefit from preloading using named parameters.
+
+__NOTE__: Although these middleware functions are added via a particular router, _when_
+they run is defined by the path they are attached to (not the router). Therefore a piece
+of middleware added via one router may run for routes of other routers if its routes
+match. For example, if two different routers are mounted on the same path:
+
+{% highlight js %}
+var authRouter = express.Router();
+var openRouter = express.Router();
+
+authRouter.use(require('./authenticate').basic(usersdb));
+
+authRouter.get('/:user_id/edit', function(req, res, next) { ... edit user ui ... });
+openRouter.get('/', function(req, res, next) { ... list users ... })
+openRouter.get('/:user_id', function(req, res, next) { ... view user ... })
+
+app.use('/users', authRouter);
+app.use('/users', openRouter);
+{% endhighlight %}
+
+Even though the authentication middleware was added via the `authRouter` it will run on the routes defined by the `openRouter` as well since both routers were mounted on `/users`.
+
+If this behavior is not desired then the paths must be different for each router.