diff --git a/book/source/conf.py b/book/source/conf.py
index 80f4181..c451430 100644
--- a/book/source/conf.py
+++ b/book/source/conf.py
@@ -32,4 +32,5 @@ html_static_path = ['_static']
 # https://github.com/sphinx-doc/alabaster/blob/0.x/alabaster/theme.conf
 html_theme_options = {
     'code_font_size': '9pt',
+    'show_relbars': 'yes',
 }