diff --git a/11_classes/05_summary.ipynb b/11_classes/05_summary.ipynb
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+    "# Chapter 11: Classes & Instances (TL;DR)"
+   ]
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+    "With the `class` statement, we can create a *user-defined* data type that we also call a **class**.\n",
+    "\n",
+    "Then, to create new **instances** of the data type, we simply call the class, just as we do with the built-in constructors.\n",
+    "\n",
+    "Conceptually, a class is the **blueprint** defining the **behavior** each instance exhibits.\n",
+    "\n",
+    "In the example used throughout the chapter, the `Vector` and `Matrix` classes implement the linear algebra rules all `Vector` and `Matrix` instances follow *in general*. On the contrary, the instances **encapsulate** the **state** of *concrete* vectors and matrices.\n",
+    "\n",
+    "The `class` statement acts as a *namespace* that consists of simple *variable assignments* and *function definitions*:\n",
+    "1. Variables become the **class attributes** that are shared among all instances.\n",
+    "2. Functions may take a different role:\n",
+    "   - By default, they become **instance methods** by going through a **binding process** where a reference to the instance on which the method is *invoked* is passed in as the first argument. By convention, the corresponding parameter is called `self`; it embodies an instance's state: That means that instance methods set and get **instance attributes** on and from `self`.\n",
+    "   - They may be declared as **class methods**. Then, the binding process is adjusted such that the first argument passed in is a reference to the class itself and, by convention, named `cls`. A common use case is to design **alternative constructors**.\n",
+    "   - They may also be declared as **properties**. A use case for that are *derived* attributes that follow semantically from an instance's state.\n",
+    "   \n",
+    "The **Python Data Model** concerns what special methods (i.e., the ones with the dunder names) exists and how they work together.\n",
+    "\n",
+    "The instantiation process is controlled by the `.__init__()` method.\n",
+    "\n",
+    "The `__repr__()` and `__str__()` methods implement the **text representation** of an instance, which can be regarded as a Unicode encoded representation of all the state encapsulated in an instance.\n",
+    "\n",
+    "**Sequence emulation** means that a user-defined data type exhibits the same four properties as the built-in sequences, which are regarded as finite and iterable containers with a predictable order. The `.__len__()`, `.__iter__()`, `__reversed__()`, `__getitem__()`, and some others are used to implement the corresponding behaviors.\n",
+    "\n",
+    "Similarly, **number emulation** means that an instance of a user-defined data type behaves like a built-in number. For example, by implementing the `.__abs__()` method, an instance may be passed to the built-in [abs() <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/library/functions.html#abs) function.\n",
+    "\n",
+    "If different data types, built-in or user-defined, share a well-defined set of behaviors, a single function may be written to work with objects of all the data types. We describe such functions as **polymorphic**.\n",
+    "\n",
+    "Classes may specify how *operators* are **overloaded**. Examples for that are the `.__add__()`, `.__sub__()`, or `.__eq__()` methods.\n",
+    "\n",
+    "**Packages** are folders containing **modules** (i.e., \\*.py files) and a \"*\\_\\_init\\_\\_.py*\" file. We use them to design coherent libraries with reusable code."
+   ]
+  }
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diff --git a/CONTENTS.md b/CONTENTS.md
index be3d163..b545cb4 100644
--- a/CONTENTS.md
+++ b/CONTENTS.md
@@ -289,3 +289,4 @@ If this is not possible,
       (Writing one's own Packages;
        The final `Vector` & `Matrix` Classes;
        Comparison with `numpy`)
+    - [summary <img height="12" style="display: inline-block" src="static/link/to_nb.png">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/11_classes/05_summary.ipynb)