diff --git a/reference/src/SUMMARY.md b/reference/src/SUMMARY.md
index 3dcb465b..bf52b9ee 100644
--- a/reference/src/SUMMARY.md
+++ b/reference/src/SUMMARY.md
@@ -11,5 +11,7 @@
   - [Function pointers](./layout/function-pointers.md)
   - [Arrays and Slices](./layout/arrays-and-slices.md)
   - [Packed SIMD vectors](./layout/packed-simd-vectors.md)
+- [Validity](./validity.md)
+  - [Unions](./validity/unions.md)
 - [Optimizations](./optimizations.md)
   - [Return value optimization](./optimizations/return_value_optimization.md)
diff --git a/reference/src/validity/unions.md b/reference/src/validity/unions.md
new file mode 100644
index 00000000..86c95478
--- /dev/null
+++ b/reference/src/validity/unions.md
@@ -0,0 +1,13 @@
+# Validity of unions
+
+**Disclaimer**: This chapter is a work-in-progress. What's contained here
+represents the consensus from issue [#73]. The statements in here are not (yet)
+"guaranteed" not to change until an RFC ratifies them.
+
+## Validity of unions with zero-sized fields
+
+A union containing a zero-sized field can contain any bit pattern. An example of such
+an union is [`MaybeUninit`].
+
+[#73]: https://github.com/rust-lang/unsafe-code-guidelines/issues/73
+[`MaybeUninit`]: https://doc.rust-lang.org/std/mem/union.MaybeUninit.html