diff -u b/core/lib/Drupal/Core/Entity/EntityType.php b/core/lib/Drupal/Core/Entity/EntityType.php
--- b/core/lib/Drupal/Core/Entity/EntityType.php
+++ b/core/lib/Drupal/Core/Entity/EntityType.php
@@ -87,7 +87,7 @@
   protected $permission_granularity;
 
   /**
-   * Indicates whether fields can be attached to entities of this type.
+   * Indicates whether fields can be extendable.
    *
    * @var bool (optional)
    */
diff -u b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
--- b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
@@ -378,7 +378,7 @@
    * Indicates whether fields can be attached to entities of this type.
    *
    * @return bool
-   *   Returns TRUE if the entity type can has fields, otherwise FALSE.
+   *   Returns TRUE if the entity can extendable, otherwise FALSE.
    */
   public function isExtendable();
 
reverted:
--- b/core/lib/Drupal/Core/Entity/FieldableDatabaseStorageController.php
+++ a/core/lib/Drupal/Core/Entity/FieldableDatabaseStorageController.php
@@ -455,7 +455,7 @@
     $queried_entities = $this->mapFromStorageRecords($queried_entities);
 
     // Attach field values.
+    if ($this->entityType->isFieldable()) {
-    if ($this->entityType->isExtendable()) {
       $this->loadFieldItems($queried_entities);
     }
 
diff -u b/core/modules/comment/comment.module b/core/modules/comment/comment.module
--- b/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -890,7 +890,7 @@
   // mismatched types. So, we need to verify that the ID is numeric (even for an
   // entity type that has an integer ID, $entity->id() might be a string
   // containing a number), and then cast it to an integer when querying.
-  if ($entity->getEntityType()->isFieldable() && is_numeric($entity->id())) {
+  if ($entity->getEntityType()->isExtendable() && is_numeric($entity->id())) {
     $cids = db_select('comment', 'c')
       ->fields('c', array('cid'))
       ->condition('entity_id', (int) $entity->id())
@@ -904,7 +904,7 @@
   // mismatched types. So, we need to verify that the ID is numeric (even for an
   // entity type that has an integer ID, $entity->id() might be a string
   // containing a number), and then cast it to an integer when querying.
-  if ($entity->getEntityType()->isExtendable() && is_numeric($entity->id())) {
+  if ($entity->getEntityType()->isFieldable() && is_numeric($entity->id())) {
     $cids = db_select('comment', 'c')
       ->fields('c', array('cid'))
       ->condition('entity_id', (int) $entity->id())
diff -u b/core/modules/field_ui/field_ui.module b/core/modules/field_ui/field_ui.module
--- b/core/modules/field_ui/field_ui.module
+++ b/core/modules/field_ui/field_ui.module
@@ -60,8 +60,8 @@
   $permissions = array();
 
   foreach (\Drupal::entityManager()->getDefinitions() as $entity_type_id => $entity_type) {
-    if ($entity_type->isFieldable()) {
-      // Create a permission for each fieldable entity to manage
+    if ($entity_type->isExtendable()) {
+      // Create a permission for each extendable entity to manage
       // the fields and the display.
       $permissions['administer ' . $entity_type_id . ' fields'] = array(
         'title' => t('%entity_label: Administer fields', array('%entity_label' => $entity_type->getLabel())),
@@ -73,8 +73,8 @@
   $permissions = array();
 
   foreach (\Drupal::entityManager()->getDefinitions() as $entity_type_id => $entity_type) {
-    if ($entity_type->isExtendable()) {
-      // Create a permission for each extendable entity to manage
+    if ($entity_type->isFieldable()) {
+      // Create a permission for each fieldable entity to manage
       // the fields and the display.
       $permissions['administer ' . $entity_type_id . ' fields'] = array(
         'title' => t('%entity_label: Administer fields', array('%entity_label' => $entity_type->getLabel())),
diff -u b/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php b/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
--- b/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
+++ b/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
@@ -87,7 +87,7 @@
     $data = array();
 
     foreach ($this->entityManager->getDefinitions() as $entity_type) {
-      if (in_array('isFieldable', get_class_methods($entity_type))) {
+      if (in_array('isExtendable', get_class_methods($entity_type))) {
         foreach ($this->entityManager->getBundleInfo($entity_type->id()) as $bundle => $bundle_info) {
           foreach ($this->entityManager->getFieldDefinitions($entity_type->id(), $bundle) as $field_definition) {
             $relation_uri = $this->getRelationUri($entity_type->id(), $bundle, $field_definition->getName());
diff -u b/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
--- b/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -746,7 +746,7 @@
  * Functions to maintain taxonomy indexing.
  *
  * Taxonomy uses default field storage to store canonical relationships
- * between terms and fieldable entities. However its most common use case
+ * between terms and extendable entities. However its most common use case
  * requires listing all content associated with a term or group of terms
  * sorted by creation date. To avoid slow queries due to joining across
  * multiple node and field tables with various conditions and order by criteria,
@@ -792,7 +792,7 @@
  * Functions to maintain taxonomy indexing.
  *
  * Taxonomy uses default field storage to store canonical relationships
- * between terms and extendable entities. However its most common use case
+ * between terms and fieldable entities. However its most common use case
  * requires listing all content associated with a term or group of terms
  * sorted by creation date. To avoid slow queries due to joining across
  * multiple node and field tables with various conditions and order by criteria,
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Entity/ContentEntityDatabaseStorage.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityDatabaseStorage.php
@@ -455,7 +455,7 @@ protected function postLoad(array &$queried_entities) {
     $queried_entities = $this->mapFromStorageRecords($queried_entities);
 
     // Attach field values.
-    if ($this->entityType->isFieldable()) {
+    if ($this->entityType->isExtendable()) {
       $this->loadFieldItems($queried_entities);
     }