diff --git a/09_mappings/06_summary.ipynb b/09_mappings/06_summary.ipynb
new file mode 100644
index 0000000..0b9c814
--- /dev/null
+++ b/09_mappings/06_summary.ipynb
@@ -0,0 +1,77 @@
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+   "source": [
+    "# Chapter 9: Mappings & Sets (TL;DR)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
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+    "`dict` objects are **mutable** one-to-one **mappings** from a set of **key** objects to a set of **value** objects. The association between a key-value pair is also called **item**.\n",
+    "\n",
+    "The items contained in a `dict` object have **no order** that is *predictable*.\n",
+    "\n",
+    "The underlying **data structure** of the `dict` type are **hash tables**. They make key look-ups *extremely* fast by converting the items' keys into *deterministic* hash values specifiying *precisely* one of a fixed number of equally \"wide\" buckets in which an item's references are stored. A limitation is that objects used as keys must be *immutable* (for technical reasons) and *unique* (for practical reasons).\n",
+    "\n",
+    "A `set` object is a **mutable** and **unordered collection** of **immutable** objects. The `set` type mimics sets we know from math."
+   ]
+  }
+ ],
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diff --git a/CONTENTS.md b/CONTENTS.md
index 76c1a8b..28dcbc1 100644
--- a/CONTENTS.md
+++ b/CONTENTS.md
@@ -258,3 +258,4 @@ If this is not possible,
       (`defaultdict` Type;
        `Counter` Type;
        `ChainMap` Type)
+    - [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/09_mappings/06_summary.ipynb)