Document adding member route without the option

1 files changed, 3 insertions, 1 deletions

diff --git a/guides/source/routing.md b/guides/source/routing.md
index 241a7cfec6..5bf31a1583 100644
--- a/guides/source/routing.md
+++ b/guides/source/routing.md
@@ -471,7 +471,7 @@ resources :photos do
 end
 ```
 
-This will recognize `/photos/1/preview` with GET, and route to the `preview` action of `PhotosController`. It will also create the `preview_photo_url` and `preview_photo_path` helpers.
+This will recognize `/photos/1/preview` with GET, and route to the `preview` action of `PhotosController`, with the resource id value passed in `params[:id]`. It will also create the `preview_photo_url` and `preview_photo_path` helpers.
 
 Within the block of member routes, each route name specifies the HTTP verb that it will recognize. You can use `get`, `patch`, `put`, `post`, or `delete` here. If you don't have multiple `member` routes, you can also pass `:on` to a route, eliminating the block:
 
@@ -481,6 +481,8 @@ resources :photos do
 end
 ```
 
+You can leave out the `:on` option, this will create the same member route except that the resource id value will be available in `params[:photo_id]` instead of `params[:id]`.
+
 #### Adding Collection Routes
 
 To add a route to the collection: