diff --git a/core/modules/taxonomy/taxonomy.admin.inc b/core/modules/taxonomy/taxonomy.admin.inc
index 99114a6420242b776b730c27fc4e13020a568841..8541b783ebd4ccbc79b3f7767f5b6c41fc98b3ab 100644
--- a/core/modules/taxonomy/taxonomy.admin.inc
+++ b/core/modules/taxonomy/taxonomy.admin.inc
@@ -115,7 +115,7 @@ function taxonomy_form_vocabulary($form, &$form_state, $edit = array()) {
       'name' => '',
       'machine_name' => '',
       'description' => '',
-      'hierarchy' => 0,
+      'hierarchy' => TAXONOMY_HIERARCHY_DISABLED,
       'weight' => 0,
     );
     foreach ($defaults as $key => $value) {
@@ -368,7 +368,7 @@ function taxonomy_overview_terms($form, &$form_state, $vocabulary) {
     }
 
     $form[$key]['view'] = array('#type' => 'link', '#title' => $term->name, '#href' => "taxonomy/term/$term->tid");
-    if ($vocabulary->hierarchy < 2 && count($tree) > 1) {
+    if ($vocabulary->hierarchy != TAXONOMY_HIERARCHY_MULTIPLE && count($tree) > 1) {
       $form['#parent_fields'] = TRUE;
       $form[$key]['tid'] = array(
         '#type' => 'hidden',
@@ -402,7 +402,7 @@ function taxonomy_overview_terms($form, &$form_state, $vocabulary) {
   $form['#forward_step'] = $forward_step;
   $form['#empty_text'] = t('No terms available. <a href="@link">Add term</a>.', array('@link' => url('admin/structure/taxonomy/' . $vocabulary->machine_name . '/add')));
 
-  if ($vocabulary->hierarchy < 2 && count($tree) > 1) {
+  if ($vocabulary->hierarchy != TAXONOMY_HIERARCHY_MULTIPLE && count($tree) > 1) {
     $form['actions'] = array('#type' => 'actions', '#tree' => FALSE);
     $form['actions']['submit'] = array(
       '#type' => 'submit',
@@ -449,7 +449,8 @@ function taxonomy_overview_terms_submit($form, &$form_state) {
   uasort($form_state['values'], 'drupal_sort_weight');
 
   $vocabulary = $form['#vocabulary'];
-  $hierarchy = 0; // Update the current hierarchy type as we go.
+  // Update the current hierarchy type as we go.
+  $hierarchy = TAXONOMY_HIERARCHY_DISABLED;
 
   $changed_terms = array();
   $tree = taxonomy_get_tree($vocabulary->vid);
@@ -468,7 +469,7 @@ function taxonomy_overview_terms_submit($form, &$form_state) {
       $changed_terms[$term['tid']] = $term;
     }
     $weight++;
-    $hierarchy = $term['parents'][0] != 0 ? 1 : $hierarchy;
+    $hierarchy = $term['parents'][0] != 0 ? TAXONOMY_HIERARCHY_SINGLE : $hierarchy;
     $term = (array) $tree[$weight];
   }
 
@@ -495,7 +496,7 @@ function taxonomy_overview_terms_submit($form, &$form_state) {
         $term['parent'] = $values['parent'];
         $changed_terms[$term['tid']] = $term;
       }
-      $hierarchy = $term['parent'] != 0 ? 1 : $hierarchy;
+      $hierarchy = $term['parent'] != 0 ? TAXONOMY_HIERARCHY_SINGLE : $hierarchy;
       $weight++;
     }
   }
@@ -508,7 +509,7 @@ function taxonomy_overview_terms_submit($form, &$form_state) {
       $term['weight'] = $weight;
       $changed_terms[$term['tid']] = $term;
     }
-    $hierarchy = $term['parents'][0] != 0 ? 1 : $hierarchy;
+    $hierarchy = $term['parents'][0] != 0 ? TAXONOMY_HIERARCHY_SINGLE : $hierarchy;
   }
 
   // Save all updated terms.
@@ -702,7 +703,7 @@ function taxonomy_form_term($form, &$form_state, $edit = array(), $vocabulary =
     '#type' => 'fieldset',
     '#title' => t('Relations'),
     '#collapsible' => TRUE,
-    '#collapsed' => $vocabulary->hierarchy < 2,
+    '#collapsed' => ($vocabulary->hierarchy != TAXONOMY_HIERARCHY_MULTIPLE),
     '#weight' => 10,
   );
 
@@ -840,8 +841,8 @@ function taxonomy_form_term_submit($form, &$form_state) {
   }
   // If we've increased the number of parents and this is a single or flat
   // hierarchy, update the vocabulary immediately.
-  elseif ($current_parent_count > $previous_parent_count && $form['#vocabulary']->hierarchy < 2) {
-    $form['#vocabulary']->hierarchy = $current_parent_count == 1 ? 1 : 2;
+  elseif ($current_parent_count > $previous_parent_count && $form['#vocabulary']->hierarchy != TAXONOMY_HIERARCHY_MULTIPLE) {
+    $form['#vocabulary']->hierarchy = $current_parent_count == 1 ? TAXONOMY_HIERARCHY_SINGLE : TAXONOMY_HIERARCHY_MULTIPLE;
     taxonomy_vocabulary_save($form['#vocabulary']);
   }
 
diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index 191b51972fd5d0638de05917e43f4dc1c118de33..c495a1eb0a6924170f6f721be19c43bfca10c5d7 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -5,6 +5,21 @@
  * Enables the organization of content into categories.
  */
 
+/**
+ * Denotes that no term in the vocabulary has a parent.
+ */
+const TAXONOMY_HIERARCHY_DISABLED = 0;
+
+/**
+ * Denotes that one or more terms in the vocabulary has a single parent.
+ */
+const TAXONOMY_HIERARCHY_SINGLE = 1;
+
+/**
+ * Denotes that one or more terms in the vocabulary have multiple parents.
+ */
+const TAXONOMY_HIERARCHY_MULTIPLE = 2;
+
 /**
  * Users can create new terms in a free-tagging vocabulary when
  * submitting a taxonomy_autocomplete_widget. We store a term object
@@ -51,11 +66,11 @@ function taxonomy_help($path, $arg) {
     case 'admin/structure/taxonomy/%':
       $vocabulary = taxonomy_vocabulary_machine_name_load($arg[3]);
       switch ($vocabulary->hierarchy) {
-        case 0:
+        case TAXONOMY_HIERARCHY_DISABLED:
           return '<p>' . t('You can reorganize the terms in %capital_name using their drag-and-drop handles, and group terms under a parent term by sliding them under and to the right of the parent.', array('%capital_name' => drupal_ucfirst($vocabulary->name), '%name' => $vocabulary->name)) . '</p>';
-        case 1:
+        case TAXONOMY_HIERARCHY_SINGLE:
           return '<p>' . t('%capital_name contains terms grouped under parent terms. You can reorganize the terms in %capital_name using their drag-and-drop handles.', array('%capital_name' => drupal_ucfirst($vocabulary->name), '%name' => $vocabulary->name)) . '</p>';
-        case 2:
+        case TAXONOMY_HIERARCHY_MULTIPLE:
           return '<p>' . t('%capital_name contains terms with multiple parents. Drag and drop of terms with multiple parents is not supported, but you can re-enable drag-and-drop support by editing each term to include only a single parent.', array('%capital_name' => drupal_ucfirst($vocabulary->name))) . '</p>';
       }
   }
@@ -519,10 +534,11 @@ function taxonomy_taxonomy_vocabulary_update($vocabulary) {
  *
  * Checks the current parents of all terms in a vocabulary and updates the
  * vocabulary's hierarchy setting to the lowest possible level. If no term
- * has parent terms then the vocabulary will be given a hierarchy of 0.
- * If any term has a single parent then the vocabulary will be given a
- * hierarchy of 1. If any term has multiple parents then the vocabulary
- * will be given a hierarchy of 2.
+ * has parent terms then the vocabulary will be given a hierarchy of
+ * TAXONOMY_HIERARCHY_DISABLED. If any term has a single parent then the
+ * vocabulary will be given a hierarchy of TAXONOMY_HIERARCHY_SINGLE. If any
+ * term has multiple parents then the vocabulary will be given a hierarchy of
+ * TAXONOMY_HIERARCHY_MULTIPLE.
  *
  * @param $vocabulary
  *   A vocabulary object.
@@ -534,7 +550,7 @@ function taxonomy_taxonomy_vocabulary_update($vocabulary) {
  */
 function taxonomy_check_vocabulary_hierarchy($vocabulary, $changed_term) {
   $tree = taxonomy_get_tree($vocabulary->vid);
-  $hierarchy = 0;
+  $hierarchy = TAXONOMY_HIERARCHY_DISABLED;
   foreach ($tree as $term) {
     // Update the changed term with the new parent value before comparison.
     if ($term->tid == $changed_term['tid']) {
@@ -543,11 +559,11 @@ function taxonomy_check_vocabulary_hierarchy($vocabulary, $changed_term) {
     }
     // Check this term's parent count.
     if (count($term->parents) > 1) {
-      $hierarchy = 2;
+      $hierarchy = TAXONOMY_HIERARCHY_MULTIPLE;
       break;
     }
     elseif (count($term->parents) == 1 && 0 !== array_shift($term->parents)) {
-      $hierarchy = 1;
+      $hierarchy = TAXONOMY_HIERARCHY_SINGLE;
     }
   }
   if ($hierarchy != $vocabulary->hierarchy) {