diff --git a/firedrake/utility_meshes.py b/firedrake/utility_meshes.py
index 6540611752..7fa5aeec73 100644
--- a/firedrake/utility_meshes.py
+++ b/firedrake/utility_meshes.py
@@ -1860,11 +1860,11 @@ def PeriodicBoxMesh(
     The boundary surfaces are numbered as follows:
 
     * 1: plane x == 0
-    * 2: plane x == 1
+    * 2: plane x == Lx
     * 3: plane y == 0
-    * 4: plane y == 1
+    * 4: plane y == Ly
     * 5: plane z == 0
-    * 6: plane z == 1
+    * 6: plane z == Lz
 
     where periodic surfaces are regarded as interior, for which dS integral is to be used.