diff --git a/Preface.ipynb b/00_Preface.ipynb
similarity index 100%
rename from Preface.ipynb
rename to 00_Preface.ipynb
diff --git a/README.md b/README.md
index 12fd7e0..16d8171 100644
--- a/README.md
+++ b/README.md
@@ -84,7 +84,7 @@ Finally, this book is free. The cost for the books required to learn Kalman filt
 Contents
 -----
 
-[**Preface**](http://nbviewer.ipython.org/urls/raw.github.com/rlabbe/Kalman-and-Bayesian-Filters-in-Python/master/Preface.ipynb)
+[**Preface**](http://nbviewer.ipython.org/urls/raw.github.com/rlabbe/Kalman-and-Bayesian-Filters-in-Python/master/00_Preface.ipynb)
 
 Motivation for the book. Where to download, how to use.
 
diff --git a/table_of_contents.ipynb b/table_of_contents.ipynb
index 76896de..b48ccd5 100644
--- a/table_of_contents.ipynb
+++ b/table_of_contents.ipynb
@@ -1,7 +1,7 @@
 {
  "metadata": {
   "name": "",
-  "signature": "sha256:a407320cc113b4c7364a292108cd117dd3dcef0349cd2cd466f2b18fedbfe7fc"
+  "signature": "sha256:5d8d19d8c30523889e3d48a53b2a661e900557175ea9b45ac6491b6c8927f13a"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -18,7 +18,7 @@
       "Table of Contents\n",
       "-----\n",
       "\n",
-      "[**Preface**](http://nbviewer.ipython.org/urls/raw.github.com/rlabbe/Kalman-and-Bayesian-Filters-in-Python/master/Preface.ipynb)\n",
+      "[**Preface**](http://nbviewer.ipython.org/urls/raw.github.com/rlabbe/Kalman-and-Bayesian-Filters-in-Python/master/00_Preface.ipynb)\n",
       " \n",
       "Motivation behind writing the book. How to download and read the book. Requirements for IPython Notebook and Python. github links.\n",
       "\n",