diff --git a/style-guide.md b/style-guide.md
index 2a35fdc5d1..391c45b644 100644
--- a/style-guide.md
+++ b/style-guide.md
@@ -77,6 +77,18 @@ instead of                          | use
 
 See [the source](api/array_view.h) for more detailed docs.
 
+### `absl::optional<T>` as function argument
+
+`absl::optional<T>` is generally a good choice when you want to pass a
+possibly missing `T` to a function&mdash;provided of course that `T`
+is a type that it makes sense to pass by value.
+
+However, when you want to avoid pass-by-value, generally **do not pass
+`const absl::optional<T>&`; use `const T*` instead.** `const
+absl::optional<T>&` forces the caller to store the `T` in an
+`absl::optional<T>`; `const T*`, on the other hand, makes no
+assumptions about how the `T` is stored.
+
 ### sigslot
 
 sigslot is a lightweight library that adds a signal/slot language