diff --git a/lib/review/book/compilable.rb b/lib/review/book/book_unit.rb
similarity index 96%
rename from lib/review/book/compilable.rb
rename to lib/review/book/book_unit.rb
index 16b4ed5b8..8e3e0e514 100644
--- a/lib/review/book/compilable.rb
+++ b/lib/review/book/book_unit.rb
@@ -10,12 +10,18 @@
 
 module ReVIEW
   module Book
-    module Compilable
+    class BookUnit
       include TextUtils
       attr_reader :book
       attr_reader :path
       attr_accessor :content
 
+      def initialize(book, number, name = nil)
+        @book = book
+        @number = number
+        @name = name
+      end
+
       def dirname
         return nil unless @path
         File.dirname(@path)
diff --git a/lib/review/book/chapter.rb b/lib/review/book/chapter.rb
index 789b3e1b1..e69be5931 100644
--- a/lib/review/book/chapter.rb
+++ b/lib/review/book/chapter.rb
@@ -7,15 +7,13 @@
 # the GNU LGPL, Lesser General Public License version 2.1.
 # For details of the GNU LGPL, see the file "COPYING".
 #
-require 'review/book/compilable'
+require 'review/book/book_unit'
 require 'review/lineinput'
 require 'review/preprocessor'
 
 module ReVIEW
   module Book
-    class Chapter
-      include Compilable
-
+    class Chapter < BookUnit
       attr_reader :number, :book
 
       def self.mkchap(book, name, number = nil)
@@ -34,9 +32,7 @@ def self.mkchap_ifexist(book, name, number = nil)
       end
 
       def initialize(book, number, name, path, io = nil)
-        @book = book
-        @number = number
-        @name = name
+        super(book, number, name)
         @path = path
         @io = io
         @title = nil
diff --git a/lib/review/book/part.rb b/lib/review/book/part.rb
index e540452cd..9db04455a 100644
--- a/lib/review/book/part.rb
+++ b/lib/review/book/part.rb
@@ -6,13 +6,11 @@
 # the GNU LGPL, Lesser General Public License version 2.1.
 # For details of the GNU LGPL, see the file "COPYING".
 #
-require 'review/book/compilable'
+require 'review/book/book_unit'
 
 module ReVIEW
   module Book
-    class Part
-      include Compilable
-
+    class Part < BookUnit
       def self.mkpart_from_namelistfile(book, path)
         chaps = []
         File.read(path, mode: 'rt:BOM|utf-8').split.each_with_index do |name, number|
@@ -36,10 +34,8 @@ def self.mkpart(chaps)
       # if Part is dummy, `number` is nil.
       #
       def initialize(book, number, chapters, name = '', io = nil)
-        @book = book
-        @number = number
+        super(book, number, name)
         @chapters = chapters
-        @name = name
         @path = name
         @content = ''
         if io