diff --git a/core/modules/book/src/Plugin/migrate/source/d6/Book.php b/core/modules/book/src/Plugin/migrate/source/d6/Book.php
index a1ecbb6b3bb6003873326a15ff07da5243028591..86fdeeeb26c919e12bd3758cdb5abefae23c16bc 100644
--- a/core/modules/book/src/Plugin/migrate/source/d6/Book.php
+++ b/core/modules/book/src/Plugin/migrate/source/d6/Book.php
@@ -53,15 +53,15 @@ public function fields() {
       'mlid' => $this->t('Menu link ID'),
       'plid' => $this->t('Parent link ID'),
       'weight' => $this->t('Weight'),
-      'p1' => $this->t('The first mlid in the materialized path.'),
-      'p2' => $this->t('The second mlid in the materialized path.'),
-      'p3' => $this->t('The third mlid in the materialized path.'),
-      'p4' => $this->t('The fourth mlid in the materialized path.'),
-      'p5' => $this->t('The fifth mlid in the materialized path.'),
-      'p6' => $this->t('The sixth mlid in the materialized path.'),
-      'p7' => $this->t('The seventh mlid in the materialized path.'),
-      'p8' => $this->t('The eight mlid in the materialized path.'),
-      'p9' => $this->t('The nine mlid in the materialized path.'),
+      'p1' => $this->t('The first mlid in the materialized path. If N = depth, then pN must equal the mlid. If depth > 1 then p(N-1) must equal the parent link mlid. All pX where X > depth must equal zero. The columns p1 .. p9 are also called the parents.'),
+      'p2' => $this->t('The second mlid in the materialized path. See p1.'),
+      'p3' => $this->t('The third mlid in the materialized path. See p1.'),
+      'p4' => $this->t('The fourth mlid in the materialized path. See p1.'),
+      'p5' => $this->t('The fifth mlid in the materialized path. See p1.'),
+      'p6' => $this->t('The sixth mlid in the materialized path. See p1.'),
+      'p7' => $this->t('The seventh mlid in the materialized path. See p1.'),
+      'p8' => $this->t('The eighth mlid in the materialized path. See p1.'),
+      'p9' => $this->t('The ninth mlid in the materialized path. See p1.'),
     );
   }