diff --git a/src/changelog.rst b/src/changelog.rst
index dcc84327..f1195ca3 100644
--- a/src/changelog.rst
+++ b/src/changelog.rst
@@ -145,6 +145,8 @@ Language changes in Rust 1.78.0
 
 * `\`trait Trait: Auto {}\`: allow upcasting from \`dyn Trait\` to \`dyn Trait + Auto\` <https://github.com/rust-lang/rust/pull/119338>`_
 
+  * New paragraph: :p:`fls_SYnFJBhi0IWj`
+
 language changes in Rust 1.77.0
 -------------------------------
 
diff --git a/src/glossary.rst b/src/glossary.rst
index c355290c..4ada47b6 100644
--- a/src/glossary.rst
+++ b/src/glossary.rst
@@ -5057,6 +5057,14 @@ primitive representation
 :dt:`Primitive representation` is the :t:`type representation` of
 :t:`[integer type]s`.
 
+.. _fls_mk3sa7OvtJvB:
+
+principal trait
+^^^^^^^^^^^^^^^
+
+:dp:`fls_YtYOHoPaMPFX`
+The :dt:`principal trait` of :t:`trait object type` is its first :t:`trait bound`.
+
 .. _fls_v1u1mevpj0kj:
 
 private visibility
diff --git a/src/types-and-traits.rst b/src/types-and-traits.rst
index 93fa081e..80863eb7 100644
--- a/src/types-and-traits.rst
+++ b/src/types-and-traits.rst
@@ -1156,10 +1156,14 @@ Trait Object Types
 A :t:`trait object type` is a :t:`type` that implements a :t:`trait`, where the
 :t:`type` is not known at compile time.
 
+:dp:`fls_eWac7zOda3lh`
+The :t:`principal trait` of :t:`trait object type` is the first :t:`trait bound`.
+
 :dp:`fls_9z8oleh0wdel`
-The first :t:`trait bound` of a :t:`trait object type` shall denote an
-:t:`object safe` :t:`trait`. Any subsequent :t:`[trait bound]s` shall denote
-:t:`[auto trait]s`.
+The :t:`principal trait` shall denote an :t:`object safe` :t:`trait`.
+
+:dp:`fls_hJII8XYAtZeY`
+All non-:t:`principal trait` :t:`[trait bound]s` shall denote :t:`[auto trait]s`.
 
 :dp:`fls_s0oy2c8t4yz9`
 A :t:`trait object type` shall not contain :t:`[opt-out trait bound]s`.
@@ -1992,6 +1996,11 @@ occur when:
   The source :t:`type` is the :t:`never type` and the target :t:`type` is any
   :t:`type`.
 
+* :dp:`fls_SYnFJBhi0IWj`
+  The source :t:`type` is a :t:`trait object type` and the target :t:`type` is a
+  :t:`trait object type` with the same :t:`[trait bound]s` and additional
+  :t:`[auto trait]s`.
+
 :dp:`fls_iiiu2q7pym4p`
 An :t:`unsized coercion` is a :t:`type coercion` that converts a :t:`sized type`
 into an :t:`unsized type`. :t:`Unsized coercion` from a source :t:`type` to a