diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 260e4772f3..96d0635ac1 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -299,10 +299,21 @@ public function __construct($environment, $class_loader, $allow_dumping = TRUE,
   /**
    * Determine the application root directory based on assumptions.
    *
+   * This implementation assumes that the application root is two levels
+   * higher in the directory structure than the PSR-4 base directory (i.e. two
+   * levels up from the path to this class in the filesystem minus it's
+   * namespace).
+   *
    * @return string
    *   The application root.
    */
   protected static function guessApplicationRoot() {
+    // Guess the application root by:
+    //
+    // - removing the namespace directories from the path
+    // - getting the path to the directory two levels up from the path
+    //   determined in the previous step.
+    //
     return dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__))));
   }