diff --git a/amp.php b/amp.php
index 43542fb3669..8090d1e422d 100644
--- a/amp.php
+++ b/amp.php
@@ -279,28 +279,26 @@ function amp_correct_query_when_is_front_page( WP_Query $query ) {
  * and everything else has AMP unavailable:
  *
  *      add_theme_support( 'amp', array(
- *          'template_dir'       => 'amp-templates/', // Optional. In case you need to override the template as a whole.
- *          'available_callback' => function() {
- *              // @todo Warning: If a plugin or theme calls is_amp_endpoint() before parse_query() then the conditionals will not work!
- *              if ( is_single() ) {
- *                  return 'native';
- *              } elseif ( is_page() ) {
- *                  return 'paired'; // Or 'true'.
- *              } else {
- *                  return false;
- *              }
- *          },
+ *          'available_callback' => 'is_single',
  *      ) );
  *
  * Then, this will change the plugin so that it won't run in 'paired mode' with separate URLs.
- * Neither will it output the rel=amphtml link on the frontend.
+ * Neither will it output the rel=amphtml link on the frontend. To serve a site in paired mode, add the paired arg:
  *
- * Paired mode will be retained if the theme registers support for AMP with just a template_dir and no available_callback:
+ *      add_theme_support( 'amp', array(
+ *          'paired' => true,
+ *          'available_callback' => 'is_single',
+ *      ) );
+ *
+ * Paired mode will also be retained if the theme registers support for AMP with just a template_dir and no available_callback:
  *
  *     add_theme_support( 'amp', array(
  *         'template_dir' => 'my-amp-templates',
  *     ) );
  *
+ * @todo Why do we even need this function?
+ * @see \AMP_Theme_Support::is_paired_available()
+ *
  * @return boolean Whether this is in AMP 'canonical' mode, that is whether it is native and there is not separate AMP URL current URL.
  */
 function amp_is_canonical() {
@@ -308,27 +306,31 @@ function amp_is_canonical() {
 	if ( true === $support ) {
 		return true;
 	}
+
 	if ( is_array( $support ) ) {
 		$args = array_shift( $support );
 
-		$is_native = (
-			isset( $args['available_callback'] )
-			&&
-			is_callable( $args['available_callback'] )
-			&&
-			'native' === call_user_func( $args['available_callback'] )
-		);
-		if ( $is_native ) {
-			return true;
+		// If paired arg is supplied, then it is never canonical.
+		if ( ! empty( $args['paired'] ) ) {
+			return false;
 		}
 
-		// If there is no available_callback and yet there is a template_dir, then paired mode is implied.
-		if ( empty( $args['available_callback'] ) && ! empty( $args['template_dir'] ) ) {
+		// If there is a template_dir, then paired mode is implied.
+		if ( ! empty( $args['template_dir'] ) ) {
 			return false;
 		}
+//
+//		// Otherwise, the available_callback dictates whether AMP is available.
+//		if ( isset( $args['available_callback'] ) && is_callable( $args['available_callback'] ) ) {
+//			return call_user_func( $args['available_callback'] );
+//		}
 	}
-	return false;
+	return true;
 }
+//
+//function amp_is_available() {
+//
+//}
 
 /**
  * Load classes.
@@ -460,6 +462,7 @@ function _amp_bootstrap_customizer() {
 
 /**
  * Redirects the old AMP URL to the new AMP URL.
+ *
  * If post slug is updated the amp page with old post slug will be redirected to the updated url.
  *
  * @since 0.5
diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index a5cc1e25626..254c264f993 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -243,10 +243,6 @@ function amp_add_amphtml_link() {
  * @return bool Whether the post supports AMP.
  */
 function post_supports_amp( $post ) {
-	if ( amp_is_canonical() ) {
-		return true;
-	}
-
 	$errors = AMP_Post_Type_Support::get_support_errors( $post );
 
 	// Return false if an error is found.
@@ -261,9 +257,9 @@ function post_supports_amp( $post ) {
 		case AMP_Post_Meta_Box::DISABLED_STATUS:
 			return false;
 
-		// Disabled by default for custom page templates, page on front and page for posts.
+		// Disabled by default for custom page templates, page on front and page for posts, unless 'amp' theme support is present.
 		default:
-			$enabled = (
+			$enabled = current_theme_supports( 'amp' ) || (
 				! (bool) get_page_template_slug( $post )
 				&&
 				! (
@@ -291,10 +287,12 @@ function post_supports_amp( $post ) {
 }
 
 /**
- * Are we currently on an AMP URL?
+ * Determine whether the current response being served as AMP.
  *
- * @since 1.0 This function can be called before the `parse_query` action because the 'amp' query var is specifically and exclusively used when 'amp' theme support is added.
+ * This function cannot be called before the parse_query action because it needs to be able
+ * to determine the queried object is able to be served as AMP.
  *
+ * @see is_header_video_active()
  * @return bool Whether it is the AMP endpoint.
  */
 function is_amp_endpoint() {
@@ -302,17 +300,47 @@ function is_amp_endpoint() {
 		return false;
 	}
 
-	// When 'amp' theme support is (or will be added) then these are the conditions that are key to be checked.
-	if ( amp_is_canonical() || isset( $_GET[ amp_get_slug() ] ) ) { // WPCS: CSRF OK.
-		return true;
+	$did_parse_query = did_action( 'parse_query' );
+
+	if ( ! $did_parse_query ) {
+		_doing_it_wrong( __FUNCTION__, sprintf( esc_html__( "is_amp_endpoint() was called before the 'parse_query' hook was called. This function will always return 'false' before the 'parse_query' hook is called.", 'amp' ) ), '0.4.2' );
+	}
+
+	$has_amp_query_var = (
+		isset( $_GET[ amp_get_slug() ] ) // WPCS: CSRF OK.
+		||
+		false !== get_query_var( amp_get_slug(), false )
+	);
+
+	// When there is no query var and AMP is not canonical/native, then this is definitely not an AMP endpoint.
+	if ( ! $has_amp_query_var && ! amp_is_canonical() ) {
+		return false;
 	}
 
-	// Condition for non-theme support when /amp/ endpoint is used.
-	if ( false !== get_query_var( amp_get_slug(), false ) ) {
-		return true;
+	/*
+	 * If theme suport is present and there is available_callback, it is what determines whether this is
+	 * an AMP endpoint. The available_callback is is presumed to check post_supports_amp().
+	 * @todo Should available_callback take a WP_Query as its arg? We need to be able to determine whether AMP is supported for another URL.
+	 * @todo Should this not be a filter?
+	 */
+	if ( current_theme_supports( 'amp' ) ) {
+		$args = get_theme_support( 'amp' );
+		if ( isset( $args[0]['available_callback'] ) && is_callable( $args[0]['available_callback'] ) ) {
+			return call_user_func( $args[0]['available_callback'] );
+		}
+	}
+
+	// When there is no theme support, then
+	if ( $did_parse_query ) {
+		$queried_object = get_queried_object();
+		return (
+			is_singular() && $queried_object instanceof WP_Post && post_supports_amp( $queried_object )
+			||
+			! is_singular() && AMP_Options_Manager::get_option( 'non_singular_supported' )
+		);
 	}
 
-	return false;
+	return true;
 }
 
 /**
diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index a43189097ab..36cec33def1 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -281,6 +281,7 @@ public static function redirect_ampless_url( $exit = true ) {
 	 * Returns true when there is a template_dir defined in theme support, and if a defined available_callback
 	 * returns true.
 	 *
+	 * @see amp_is_canonical()
 	 * @return bool Whether available.
 	 */
 	public static function is_paired_available() {
@@ -299,6 +300,10 @@ public static function is_paired_available() {
 			return false;
 		}
 
+		if ( ! is_singular() && ! AMP_Options_Manager::get_option( 'non_singular_supported' ) ) {
+			return false;
+		}
+
 		$args = array_shift( $support );
 
 		if ( isset( $args['available_callback'] ) && is_callable( $args['available_callback'] ) ) {