diff --git a/core/lib/Drupal/Core/Render/theme.api.php b/core/lib/Drupal/Core/Render/theme.api.php
index 44fcdd0..3b92f2a 100644
--- a/core/lib/Drupal/Core/Render/theme.api.php
+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -607,31 +607,53 @@ function hook_preprocess_HOOK(&$variables) {
 }
 
 /**
- * Provides alternate named suggestions for a specific theme hook.
+ * Provides more specific templates or theme functions for a theme hook.
  *
- * This hook allows modules to provide alternative theme function or template
- * name suggestions.
+ * These alternative templates or theme functions can optionally be used by a
+ * theme to override markup or display logic based on incoming variables or
+ * global context.
  *
- * HOOK is the least-specific version of the hook being called. For example, if
- * '#theme' => 'node__article' is called, then hook_theme_suggestions_node()
- * will be invoked, not hook_theme_suggestions_node__article(). The specific
- * hook called (in this case 'node__article') is available in
- * $variables['theme_hook_original'].
+ * For a specific theme hook like 'node', this hook allows modules to provide
+ * more specific template name suggestions, for example node--front.html.twig
+ * for a node displayed on the front page, or node--article.html.twig for an
+ * article node.
  *
- * @todo Add @code sample.
+ * HOOK is the least-specific version of the hook being called, also known as
+ * the base theme hook. For example, if '#theme' => 'node__article' is called,
+ * then hook_theme_suggestions_node() will be invoked, not
+ * hook_theme_suggestions_node__article(). The specific hook called (in this
+ * case 'node__article') is available in $variables['theme_hook_original'].
+ *
+ * Note that this hook is not invoked for themes, even if the theme implements
+ * hook_theme(). Themes must use hook_theme_suggestions_HOOK_alter().
+ *
+ * The following example provides an optional block--not-front.html.twig
+ * template that could be used to change the markup or display logic for blocks
+ * shown on every page but the front page:
+ * @code
+ * function block_theme_suggestions_block(array $variables) {
+ *   $suggestions = array();
+ *   if (!drupal_is_front_page()) {
+ *     $suggestions[] = 'block__not_front';
+ *   }
+ *   return $suggestions;
+ * }
+ * @endcode
  *
  * @param array $variables
  *   An array of variables passed to the theme hook. Note that this hook is
- *   invoked before any preprocessing.
+ *   invoked before any variable preprocessing.
  *
  * @return array
- *   An array of theme suggestions.
+ *   An array of alternate, more specific names for template files or theme
+ *   functions.
  *
  * @see hook_theme_suggestions_HOOK_alter()
  */
 function hook_theme_suggestions_HOOK(array $variables) {
-  $suggestions = [];
-
+  $suggestions = array();
+  // If the node language is French, this would provide node--fr.html.twig as an
+  // optional template override for French nodes.
   $suggestions[] = 'node__' . $variables['elements']['#langcode'];
 
   return $suggestions;
@@ -687,25 +709,34 @@ function hook_theme_suggestions_alter(array &$suggestions, array $variables, $ho
 }
 
 /**
- * Alters named suggestions for a specific theme hook.
+ * Alters available templates or theme function names for a theme hook.
  *
  * This hook allows any module or theme to provide alternative theme function or
  * template name suggestions and reorder or remove suggestions provided by
  * hook_theme_suggestions_HOOK() or by earlier invocations of this hook.
  *
- * HOOK is the least-specific version of the hook being called. For example, if
- * '#theme' => 'node__article' is called, then node_theme_suggestions_node()
- * will be invoked, not node_theme_suggestions_node__article(). The specific
- * hook called (in this case 'node__article') is available in
- * $variables['theme_hook_original'].
+ * HOOK is the least-specific version of the hook being called, also known as
+ * the base theme hook. For example, if '#theme' => 'node__article' is called,
+ * then node_theme_suggestions_node() will be invoked, not
+ * node_theme_suggestions_node__article(). The specific hook called (in this
+ * case 'node__article') is available in $variables['theme_hook_original'].
  *
- * @todo Add @code sample.
+ * The following example provides an optional alternative template of
+ * node--with-image.html.twig for nodes that an 'image' field with a value:
+ * @code
+ * function MYMODULE_theme_suggestions_node_alter(array &$suggestions, array $variables) {
+ *   $node = $variables['elements']['#node'];
+ *   if ($node->hasField('field_image') && $node->field_image->getValue()) {
+ *     $suggestions[] = 'node__with_image';
+ *   }
+ * }
+ * @endcode
  *
  * @param array $suggestions
  *   An array of theme suggestions.
  * @param array $variables
  *   An array of variables passed to the theme hook. Note that this hook is
- *   invoked before any preprocessing.
+ *   invoked before any variable preprocessing.
  *
  * @see hook_theme_suggestions_alter()
  * @see hook_theme_suggestions_HOOK()
diff --git a/core/modules/system/src/Tests/Theme/ThemeSuggestionsAlterTest.php b/core/modules/system/src/Tests/Theme/ThemeSuggestionsAlterTest.php
index c01a94f..c5050bf 100644
--- a/core/modules/system/src/Tests/Theme/ThemeSuggestionsAlterTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeSuggestionsAlterTest.php
@@ -64,6 +64,43 @@ public function testGeneralSuggestionsAlter() {
   }
 
   /**
+   * Tests that theme suggestion alter hooks work with an array of theme hooks.
+   */
+  public function testArraySuggestionsAlter() {
+    $this->drupalGet('theme-test/array-suggestions-alter');
+    $this->assertText('Template for testing suggestion alter hooks with an array of theme suggestions.');
+
+    // Enable test_theme, it contains a hook_theme_suggestions_HOOK_alter()
+    // implementation and template that we can test.
+    $this->config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
+
+    $this->drupalGet('theme-test/array-suggestions-alter');
+    $this->assertText('Template overridden by adding a new suggestion via test_theme_theme_suggestions_theme_test_array_suggestions_alter().');
+  }
+
+  /**
+   * Tests that theme suggestion alter hooks work with an array of theme hooks.
+   *
+   * In this example no base hook is specified in the array of theme hooks/theme
+   * suggestions.
+   */
+  public function testArraySuggestionsWithoutBaseAlter() {
+    $this->drupalGet('theme-test/array-suggestions-without-base-alter');
+    $this->assertText('Template for testing suggestion alter hooks with an array of theme suggestions without specifying a base hook.');
+
+    // Enable test_theme, it contains a hook_theme_suggestions_HOOK_alter()
+    // implementation and template that we can test.
+    $this->config('system.theme')
+      ->set('default', 'test_theme')
+      ->save();
+
+    $this->drupalGet('theme-test/array-suggestions-without-base-alter');
+    $this->assertText('Template overridden by adding a new suggestion via test_theme_theme_suggestions_theme_test_array_suggestions_without_base_alter().');
+  }
+
+  /**
    * Tests that theme suggestion alter hooks work for templates.
    */
   public function testTemplateSuggestionsAlter() {
diff --git a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
index f1ed8f3..1533676 100644
--- a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
@@ -159,4 +159,32 @@ public function preprocessSuggestions() {
     ];
   }
 
+  /**
+   * Menu callback for testing suggestion alter hooks with an array of hooks.
+   *
+   * In this example the base hook is specified as the first suggestion.
+   */
+  public function arraySuggestionsAlter() {
+    return array('#theme' => array(
+      'theme_test_array_suggestions',
+      'theme_test_array_suggestions__first_suggestion',
+      'theme_test_array_suggestions__second_suggestion',
+    ));
+  }
+
+  /**
+   * Menu callback for testing suggestion alter hooks with an array of hooks.
+   *
+   * In this example no base hook is specified, but if no suggestions have been
+   * implemented as templates or theme functions, theme() will automatically
+   * determine the base hook, in this case
+   * 'theme_test_array_suggestions_without_base'.
+   */
+  public function arraySuggestionsWithoutBaseAlter() {
+    return array('#theme' => array(
+      'theme_test_array_suggestions_without_base__first_suggestion',
+      'theme_test_array_suggestions_without_base__second_suggestion',
+    ));
+  }
+
 }
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions-without-base--first-suggestion.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions-without-base--first-suggestion.html.twig
new file mode 100644
index 0000000..c0b4d04
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions-without-base--first-suggestion.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This template should not be rendered.
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions-without-base.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions-without-base.html.twig
new file mode 100644
index 0000000..1f1e463
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions-without-base.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template for testing suggestion alter hooks with an array of theme suggestions without specifying a base hook.
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions.html.twig
new file mode 100644
index 0000000..ec7c2fc
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-array-suggestions.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template for testing suggestion alter hooks with an array of theme suggestions.
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 2e6f164..57a2a15 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -21,21 +21,27 @@ function theme_test_theme($existing, $type, $theme, $path) {
   ];
   $items['theme_test_template_test_2'] = [
     'template' => 'theme_test.template_test',
-  ];
-  $items['theme_test_suggestion_provided'] = [
-    'variables' => [],
-  ];
-  $items['theme_test_specific_suggestions'] = [
-    'variables' => [],
-  ];
-  $items['theme_test_suggestions'] = [
-    'variables' => [],
-  ];
-  $items['theme_test_general_suggestions'] = [
-    'variables' => [],
-  ];
-  $items['theme_test_function_suggestions'] = [
-    'variables' => [],
+  );
+  $items['theme_test_suggestion_provided'] = array(
+    'variables' => array(),
+  );
+  $items['theme_test_specific_suggestions'] = array(
+    'variables' => array(),
+  );
+  $items['theme_test_suggestions'] = array(
+    'variables' => array(),
+  );
+  $items['theme_test_general_suggestions'] = array(
+    'variables' => array(),
+  );
+  $items['theme_test_array_suggestions'] = array(
+    'variables' => array(),
+  );
+  $items['theme_test_array_suggestions_without_base'] = array(
+    'variables' => array(),
+  );
+  $items['theme_test_function_suggestions'] = array(
+    'variables' => array(),
     'function' => 'theme_theme_test_function_suggestions',
   ];
   $items['theme_test_suggestions_include'] = [
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
index 1ff61cf..faae6ea 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
@@ -55,7 +55,7 @@ theme_test.request_listener:
   requirements:
     _access: 'TRUE'
 
-suggestion_alter:
+theme_test.suggestion_alter:
   path: '/theme-test/suggestion-alter'
   defaults:
     _controller: '\Drupal\theme_test\ThemeTestController::suggestionAlter'
@@ -69,21 +69,21 @@ theme_test.general_suggestion_alter:
   requirements:
     _access: 'TRUE'
 
-suggestion_provided:
+theme_test.suggestion_provided:
   path: '/theme-test/suggestion-provided'
   defaults:
     _controller: '\Drupal\theme_test\ThemeTestController::suggestionProvided'
   requirements:
     _access: 'TRUE'
 
-specific_suggestion_alter:
+theme_test.specific_suggestion_alter:
   path: '/theme-test/specific-suggestion-alter'
   defaults:
     _controller: '\Drupal\theme_test\ThemeTestController::specificSuggestionAlter'
   requirements:
     _access: 'TRUE'
 
-function_suggestion_alter:
+theme_test.function_suggestion_alter:
   path: '/theme-test/function-suggestion-alter'
   defaults:
     _controller: '\Drupal\theme_test\ThemeTestController::functionSuggestionAlter'
@@ -110,3 +110,17 @@ theme_test.preprocess_suggestions:
     _controller: '\Drupal\theme_test\ThemeTestController::preprocessSuggestions'
   requirements:
     _access: 'TRUE'
+
+theme_test.array_suggestions_alter:
+  path: '/theme-test/array-suggestions-alter'
+  defaults:
+    _controller: '\Drupal\theme_test\ThemeTestController::arraySuggestionsAlter'
+  requirements:
+    _access: 'TRUE'
+
+theme_test.array_suggestions_without_base_alter:
+  path: '/theme-test/array-suggestions-without-base-alter'
+  defaults:
+    _controller: '\Drupal\theme_test\ThemeTestController::arraySuggestionsWithoutBaseAlter'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--first-suggestion.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--first-suggestion.html.twig
new file mode 100644
index 0000000..c0b4d04
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--first-suggestion.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This template should not be rendered.
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--other-suggestion.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--other-suggestion.html.twig
new file mode 100644
index 0000000..c653e56
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--other-suggestion.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template overridden by adding a new suggestion via test_theme_theme_suggestions_theme_test_array_suggestions_alter().
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--second-suggestion.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--second-suggestion.html.twig
new file mode 100644
index 0000000..c0b4d04
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions--second-suggestion.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+This template should not be rendered.
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions-without-base--other-suggestion.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions-without-base--other-suggestion.html.twig
new file mode 100644
index 0000000..a492623
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-array-suggestions-without-base--other-suggestion.html.twig
@@ -0,0 +1,2 @@
+{# Output for Theme API test #}
+Template overridden by adding a new suggestion via test_theme_theme_suggestions_theme_test_array_suggestions_without_base_alter().
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index aa399b6..99ba45b 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -87,6 +87,22 @@ function test_theme_theme_suggestions_theme_test_function_suggestions_alter(arra
 }
 
 /**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function test_theme_theme_suggestions_theme_test_array_suggestions_alter(array &$suggestions, array $variables) {
+  // Add an alternative suggestion to the end.
+  $suggestions[] = 'theme_test_array_suggestions__' . 'other_suggestion';
+}
+
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function test_theme_theme_suggestions_theme_test_array_suggestions_without_base_alter(array &$suggestions, array $variables) {
+  // Add an alternative suggestion to the end.
+  $suggestions[] = 'theme_test_array_suggestions_without_base__' . 'other_suggestion';
+}
+
+/**
  * Returns HTML for a theme function suggestion test.
  *
  * Implements the theme_test_function_suggestions__theme_override suggestion.