diff --git a/core/includes/form.inc b/core/includes/form.inc
index 8bd89fb..8ead9e4 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -7,7 +7,7 @@
 
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Form\FormInterface;
-use Drupal\Core\Form\BaseFormInterface;
+use Drupal\Core\Form\BaseFormIdInterface;
 use Drupal\Core\Database\Database;
 use Drupal\Core\Template\Attribute;
 use Drupal\Core\Datetime\DrupalDateTime;
@@ -120,7 +120,7 @@ function _drupal_form_id($form_arg, &$form_state) {
   // the callback object and determine the form ID.
   if (is_object($form_arg) && $form_arg instanceof FormInterface) {
     $form_state['build_info']['callback_object'] = $form_arg;
-    if ($form_arg instanceof BaseFormInterface) {
+    if ($form_arg instanceof BaseFormIdInterface) {
       $form_state['build_info']['base_form_id'] = $form_arg->getBaseFormID();
     }
     return $form_arg->getFormID();
diff --git a/core/lib/Drupal/Core/Entity/EntityFormControllerInterface.php b/core/lib/Drupal/Core/Entity/EntityFormControllerInterface.php
index 28b2562..289df86 100644
--- a/core/lib/Drupal/Core/Entity/EntityFormControllerInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityFormControllerInterface.php
@@ -7,12 +7,12 @@
 
 namespace Drupal\Core\Entity;
 
-use Drupal\Core\Form\BaseFormInterface;
+use Drupal\Core\Form\BaseFormIdInterface;
 
 /**
  * Defines a common interface for entity form controller classes.
  */
-interface EntityFormControllerInterface extends BaseFormInterface {
+interface EntityFormControllerInterface extends BaseFormIdInterface {
 
   /**
    * Returns the code identifying the active form language.
diff --git a/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php b/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php
index 9177a23..8237ba1 100644
--- a/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php
+++ b/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php
@@ -18,7 +18,8 @@ class HtmlEntityFormController extends HtmlFormController {
   /**
    * {@inheritdoc}
    *
-   * Due to reflection, the argument must be named $_entity_form.
+   * Due to reflection, the argument must be named $_entity_form. The parent
+   * method has $request and $_form, but the parameter must match the route.
    */
   public function content(Request $request, $_entity_form) {
     return parent::content($request, $_entity_form);
@@ -27,7 +28,7 @@ public function content(Request $request, $_entity_form) {
   /**
    * Overrides \Drupal\Core\HtmlFormController::getFormObject().
    *
-   * Instead of a class name or service ID, $arg will be a string representing
+   * Instead of a class name or service ID, $form_arg will be a string representing
    * the entity and operation being performed.
    * Consider the following route:
    * @code
@@ -35,7 +36,7 @@ public function content(Request $request, $_entity_form) {
    *   defaults:
    *     _entity_form: 'node.edit'
    * @endcode
-   * This would mean the edit form controller for the node entity should be use.
+   * This means that the edit form controller for the node entity will used.
    * If the entity type has a default form controller, only the name of the
    * entity {param} needs to be passed:
    * @code
@@ -44,17 +45,17 @@ public function content(Request $request, $_entity_form) {
    *     _entity_form: 'node'
    * @endcode
    */
-  protected function getFormObject(Request $request, $arg) {
+  protected function getFormObject(Request $request, $form_arg) {
     $manager = $this->container->get('plugin.manager.entity');
 
     // If no operation is provided, use 'default'.
-    $arg .= '.default';
-    list ($entity, $operation) = explode('.', $arg);
+    $form_arg .= '.default';
+    list ($entity, $operation) = explode('.', $form_arg);
 
     if ($request->attributes->has($entity)) {
       $entity = $request->attributes->get($entity);
     }
-    elseif (is_string($entity)) {
+    else {
       $entity = $manager->getStorageController($entity)->create(array());
     }
 
diff --git a/core/lib/Drupal/Core/Form/BaseFormInterface.php b/core/lib/Drupal/Core/Form/BaseFormIdInterface.php
similarity index 58%
rename from core/lib/Drupal/Core/Form/BaseFormInterface.php
rename to core/lib/Drupal/Core/Form/BaseFormIdInterface.php
index fa624cd..f948aac 100644
--- a/core/lib/Drupal/Core/Form/BaseFormInterface.php
+++ b/core/lib/Drupal/Core/Form/BaseFormIdInterface.php
@@ -2,23 +2,24 @@
 
 /**
  * @file
- * Contains \Drupal\Core\Form\BaseFormInterface.
+ * Contains \Drupal\Core\Form\BaseFormIdInterface.
  */
 
 namespace Drupal\Core\Form;
 
 /**
  * Provides an interface for a Form that has a base form ID.
+ *
+ * This will become the $form_state['build_info']['base_form_id'] used to
+ * generate the name of hook_form_BASE_FORM_ID_alter().
  */
-interface BaseFormInterface extends FormInterface {
+interface BaseFormIdInterface extends FormInterface {
 
   /**
    * Returns a string identifying the base form.
    *
    * @return string|false
    *   The string identifying the base form or FALSE if this is not a base form.
-   *
-   *   The unique string identifying the form.
    */
   public function getBaseFormID();
 
diff --git a/core/lib/Drupal/Core/HtmlFormController.php b/core/lib/Drupal/Core/HtmlFormController.php
index a589396..e19d8ea 100644
--- a/core/lib/Drupal/Core/HtmlFormController.php
+++ b/core/lib/Drupal/Core/HtmlFormController.php
@@ -71,24 +71,24 @@ public function content(Request $request, $_form) {
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The request using this form.
-   * @param string $arg
+   * @param string $form_arg
    *   Either a class name or a service ID.
    *
    * @return \Drupal\Core\Form\FormInterface
    *   The form object to use.
    */
-  protected function getFormObject(Request $request, $arg) {
+  protected function getFormObject(Request $request, $form_arg) {
     // If this is a class, instantiate it.
-    if (class_exists($arg)) {
-      if (in_array('Drupal\Core\ControllerInterface', class_implements($arg))) {
-        return $arg::create($this->container);
+    if (class_exists($form_arg)) {
+      if (in_array('Drupal\Core\ControllerInterface', class_implements($form_arg))) {
+        return $form_arg::create($this->container);
       }
 
-      return new $arg();
+      return new $form_arg();
     }
 
     // Otherwise, it is a service.
-    return $this->container->get($arg);
+    return $this->container->get($form_arg);
   }
 
 }