diff --git a/book/src/binding/str.md b/book/src/binding/str.md
index 9c1e0a773..e7b9f6e0e 100644
--- a/book/src/binding/str.md
+++ b/book/src/binding/str.md
@@ -28,7 +28,9 @@ public:
 
   // Note: no null terminator.
   const char *data() const noexcept;
+  // Length in bytes
   size_t size() const noexcept;
+  // Length in bytes (alias for `size()`)
   size_t length() const noexcept;
   bool empty() const noexcept;