diff -u b/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
--- b/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -1090,9 +1090,10 @@
    */
   protected function initializeRequestGlobals(Request $request) {
     global $base_url;
+    // Set and derived from $base_url by this function.
     global $base_path, $base_root;
     global $base_secure_url, $base_insecure_url;
-
+    // Create base URL.
     $base_path = $request->getBasePath();
     // Requests to 'core/foo.php' bypass the front controller, causing
     // $request->getBasePath() to return a string ending with '/core'. In these
diff -u b/core/tests/Drupal/Tests/Core/DrupalKernel/DrupalKernelTest.php b/core/tests/Drupal/Tests/Core/DrupalKernel/DrupalKernelTest.php
--- b/core/tests/Drupal/Tests/Core/DrupalKernel/DrupalKernelTest.php
+++ b/core/tests/Drupal/Tests/Core/DrupalKernel/DrupalKernelTest.php
@@ -13,7 +13,7 @@
    * @group DrupalKernel
    */
   class DrupalKernelTest extends UnitTestCase {
-
+    //Back up and restore any global variables that may be changed by tests.
     protected $backupGlobals = FALSE;
 
     /**