diff --git a/src/applications/settings/editor/PhabricatorSettingsEditEngine.php b/src/applications/settings/editor/PhabricatorSettingsEditEngine.php
--- a/src/applications/settings/editor/PhabricatorSettingsEditEngine.php
+++ b/src/applications/settings/editor/PhabricatorSettingsEditEngine.php
@@ -7,6 +7,7 @@
 
   private $isSelfEdit;
   private $profileURI;
+  private $settingsPanel;
 
   public function setIsSelfEdit($is_self_edit) {
     $this->isSelfEdit = $is_self_edit;
@@ -26,6 +27,15 @@
     return $this->profileURI;
   }
 
+  public function setSettingsPanel($settings_panel) {
+    $this->settingsPanel = $settings_panel;
+    return $this;
+  }
+
+  public function getSettingsPanel() {
+    return $this->settingsPanel;
+  }
+
   public function isEngineConfigurable() {
     return false;
   }
@@ -252,13 +262,29 @@
   protected function newEditFormHeadContent(
     PhabricatorEditEnginePageState $state) {
 
+    $content = array();
+
     if ($state->getIsSave()) {
-      return id(new PHUIInfoView())
+      $content[] = id(new PHUIInfoView())
         ->setSeverity(PHUIInfoView::SEVERITY_NOTICE)
         ->appendChild(pht('Changes saved.'));
     }
 
-    return null;
+    $panel = $this->getSettingsPanel();
+    $content[] = $panel->newSettingsPanelEditFormHeadContent($state);
+
+    return $content;
+  }
+
+  protected function newEditFormTailContent(
+    PhabricatorEditEnginePageState $state) {
+
+    $content = array();
+
+    $panel = $this->getSettingsPanel();
+    $content[] = $panel->newSettingsPanelEditFormTailContent($state);
+
+    return $content;
   }
 
 }
diff --git a/src/applications/settings/panel/PhabricatorEditEngineSettingsPanel.php b/src/applications/settings/panel/PhabricatorEditEngineSettingsPanel.php
--- a/src/applications/settings/panel/PhabricatorEditEngineSettingsPanel.php
+++ b/src/applications/settings/panel/PhabricatorEditEngineSettingsPanel.php
@@ -22,6 +22,7 @@
     $engine = id(new PhabricatorSettingsEditEngine())
       ->setController($this->getController())
       ->setNavigation($this->getNavigation())
+      ->setSettingsPanel($this)
       ->setIsSelfEdit($is_self)
       ->setProfileURI($profile_uri);
 
@@ -71,4 +72,14 @@
     return mpull($panel_settings, 'getSettingKey');
   }
 
+  public function newSettingsPanelEditFormHeadContent(
+    PhabricatorEditEnginePageState $state) {
+    return null;
+  }
+
+  public function newSettingsPanelEditFormTailContent(
+    PhabricatorEditEnginePageState $state) {
+    return null;
+  }
+
 }
diff --git a/src/applications/settings/panel/PhabricatorExternalEditorSettingsPanel.php b/src/applications/settings/panel/PhabricatorExternalEditorSettingsPanel.php
--- a/src/applications/settings/panel/PhabricatorExternalEditorSettingsPanel.php
+++ b/src/applications/settings/panel/PhabricatorExternalEditorSettingsPanel.php
@@ -21,4 +21,150 @@
     return true;
   }
 
+  public function newSettingsPanelEditFormHeadContent(
+    PhabricatorEditEnginePageState $state) {
+
+    // The "Editor" setting stored in the database may be invalidated by
+    // configuration or software changes. If a saved URI becomes invalid
+    // (for example, its protocol is removed from the list of allowed
+    // protocols), it will stop working.
+
+    // If the stored value has a problem like this, show a static error
+    // message without requiring the user to save changes.
+
+    if ($state->getIsSubmit()) {
+      return null;
+    }
+
+    $viewer = $this->getViewer();
+    $pattern = $viewer->getUserSetting(PhabricatorEditorSetting::SETTINGKEY);
+
+    if (!strlen($pattern)) {
+      return null;
+    }
+
+    $caught = null;
+    try {
+      id(new PhabricatorEditorURIEngine())
+        ->setPattern($pattern)
+        ->validatePattern();
+    } catch (PhabricatorEditorURIParserException $ex) {
+      $caught = $ex;
+    }
+
+    if (!$caught) {
+      return null;
+    }
+
+    return id(new PHUIInfoView())
+      ->setSeverity(PHUIInfoView::SEVERITY_WARNING)
+      ->appendChild($caught->getMessage());
+  }
+
+  public function newSettingsPanelEditFormTailContent(
+    PhabricatorEditEnginePageState $state) {
+    $viewer = $this->getViewer();
+
+    $variables = PhabricatorEditorURIEngine::getVariableDefinitions();
+
+    $rows = array();
+    foreach ($variables as $key => $variable) {
+      $rows[] = array(
+        phutil_tag('tt', array(), '%'.$key),
+        $variable['name'],
+        $variable['example'],
+      );
+    }
+
+    $table = id(new AphrontTableView($rows))
+      ->setHeaders(
+        array(
+          pht('Variable'),
+          pht('Replaced With'),
+          pht('Example'),
+        ))
+      ->setColumnClasses(
+        array(
+          'center',
+          'pri',
+          'wide',
+        ));
+
+    $variables_box = id(new PHUIObjectBoxView())
+      ->setBackground(PHUIObjectBoxView::WHITE_CONFIG)
+      ->setHeaderText(pht('External Editor URI Variables'))
+      ->setTable($table);
+
+    $label_map = array(
+      'http' => pht('Hypertext Transfer Protocol'),
+      'https' => pht('Hypertext Transfer Protocol over SSL'),
+      'txmt' => pht('TextMate'),
+      'mvim' => pht('MacVim'),
+      'subl' => pht('Sublime Text'),
+      'vim' => pht('Vim'),
+      'emacs' => pht('Emacs'),
+      'vscode' => pht('Visual Studio Code'),
+      'editor' => pht('Generic Editor'),
+    );
+
+    $default_label = phutil_tag('em', array(), pht('Supported Protocol'));
+
+    $config_key = 'uri.allowed-editor-protocols';
+
+    $protocols = PhabricatorEnv::getEnvConfig($config_key);
+    $protocols = array_keys($protocols);
+    sort($protocols);
+
+    $protocol_rows = array();
+    foreach ($protocols as $protocol) {
+      $label = idx($label_map, $protocol, $default_label);
+
+      $protocol_rows[] = array(
+        $protocol.'://',
+        $label,
+      );
+    }
+
+    $protocol_table = id(new AphrontTableView($protocol_rows))
+      ->setNoDataString(
+        pht(
+          'Phabricator is not configured to allow any editor protocols.'))
+      ->setHeaders(
+        array(
+          pht('Protocol'),
+          pht('Description'),
+        ))
+      ->setColumnClasses(
+        array(
+          'pri',
+          'wide',
+        ));
+
+    $is_admin = $viewer->getIsAdmin();
+
+    $configure_button = id(new PHUIButtonView())
+      ->setTag('a')
+      ->setText(pht('View Configuration'))
+      ->setIcon('fa-sliders')
+      ->setHref(
+        urisprintf(
+          '/config/edit/%s/',
+          $config_key))
+      ->setDisabled(!$is_admin);
+
+    $protocol_header = id(new PHUIHeaderView())
+      ->setHeader(pht('Supported Editor Protocols'))
+      ->addActionLink($configure_button);
+
+    $protocols_box = id(new PHUIObjectBoxView())
+      ->setBackground(PHUIObjectBoxView::WHITE_CONFIG)
+      ->setHeader($protocol_header)
+      ->setTable($protocol_table);
+
+    return array(
+      $variables_box,
+      $protocols_box,
+    );
+  }
+
 }
diff --git a/src/applications/settings/setting/PhabricatorEditorSetting.php b/src/applications/settings/setting/PhabricatorEditorSetting.php
--- a/src/applications/settings/setting/PhabricatorEditorSetting.php
+++ b/src/applications/settings/setting/PhabricatorEditorSetting.php
@@ -20,20 +20,23 @@
   protected function getControlInstructions() {
     return pht(
       "Many text editors can be configured as URI handlers for special ".
-      "protocols like `editor://`. If you have such an editor, Phabricator ".
-      "can generate links that you can click to open files locally.".
+      "protocols like `editor://`. If you have installed and configured ".
+      "such an editor, Phabricator can generate links that you can click ".
+      "to open files locally.".
       "\n\n".
-      "These special variables are supported:".
+      "Provide a URI pattern for building external editor URIs in your ".
+      "environment. For example, if you use TextMate on macOS, the pattern ".
+      "for your machine look like this:".
       "\n\n".
-      "| Value | Replaced With |\n".
-      "|-------|---------------|\n".
-      "| `%%f`  | Filename |\n".
-      "| `%%l`  | Line Number |\n".
-      "| `%%r`  | Repository Callsign |\n".
-      "| `%%%%`  | Literal `%%` |\n".
+      "```name=\"Example: TextMate on macOS\"\n".
+      "%s\n".
+      "```\n".
       "\n\n".
       "For complete instructions on editor configuration, ".
-      "see **[[ %s | %s ]]**.",
+      "see **[[ %s | %s ]]**.".
+      "\n\n".
+      "See the tables below for a list of supported variables and protocols.",
+      'txmt://open/?url=file:///Users/alincoln/editor_links/%r/%f&line=%l',
       PhabricatorEnv::getDoclink('User Guide: Configuring an External Editor'),
       pht('User Guide: Configuring an External Editor'));
   }
diff --git a/src/docs/user/userguide/external_editor.diviner b/src/docs/user/userguide/external_editor.diviner
--- a/src/docs/user/userguide/external_editor.diviner
+++ b/src/docs/user/userguide/external_editor.diviner
@@ -3,12 +3,14 @@
 
 Setting up an external editor to integrate with Diffusion and Differential.
 
-= Overview =
+Overview
+========
 
 You can configure a URI handler to allow you to open files from Differential
 and Diffusion in your preferred text editor.
 
-= Configuring Editors =
+Configuring Editors
+===================
 
 To configure an external editor, go to {nav Settings > Application Settings >
 External Editor} and set "Editor Link" to a URI pattern (see below). This
@@ -17,15 +19,12 @@
 
 In general, you'll set this field to something like:
 
-  lang=uri
-  editor://open/?file=%f
+```lang=uri
+editor://open/?file=%f
+```
 
-Some editors support opening multiple files at once when filenames are separated
-by spaces. If your editor supports this feature, set "Edit Multiple Files" to
-"Supported". Otherwise, you can set it to "Not Supported" to disable "Open All"
-buttons in the interface.
-
-== Configuring: TextMate on OS X ==
+Configuring: TextMate on macOS
+==============================
 
 TextMate installs a `txmt://` handler by default, so it's easy to configure
 this feature if you use TextMate.
@@ -40,5 +39,6 @@
 
 Then set your "Editor Link" to:
 
-  lang=uri
-  txmt://open/?url=file:///Users/alincoln/editor_links/%r/%f&line=%l
+```lang=uri
+txmt://open/?url=file:///Users/alincoln/editor_links/%r/%f&line=%l
+```
diff --git a/src/infrastructure/editor/PhabricatorEditorURIEngine.php b/src/infrastructure/editor/PhabricatorEditorURIEngine.php
--- a/src/infrastructure/editor/PhabricatorEditorURIEngine.php
+++ b/src/infrastructure/editor/PhabricatorEditorURIEngine.php
@@ -92,15 +92,19 @@
     return array(
       '%' => array(
         'name' => pht('Literal Percent Symbol'),
+        'example' => '%',
       ),
       'r' => array(
         'name' => pht('Repository Callsign'),
+        'example' => 'XYZ',
       ),
       'f' => array(
         'name' => pht('File Name'),
+        'example' => pht('path/to/source.c'),
       ),
       'l' => array(
         'name' => pht('Line Number'),
+        'example' => '777',
       ),
     );
   }