diff --git a/betty/tests/test_typing.py b/betty/tests/test_typing.py
index b878565eb..7b0b81d4f 100644
--- a/betty/tests/test_typing.py
+++ b/betty/tests/test_typing.py
@@ -1,4 +1,4 @@
-from betty.typing import internal
+from betty.typing import internal, public
 
 
 class TestInternal:
@@ -10,3 +10,14 @@ def _target() -> object:
             return sentinel
 
         assert _target() is sentinel
+
+
+class TestPublic:
+    def test(self) -> None:
+        sentinel = object()
+
+        @public
+        def _target() -> object:
+            return sentinel
+
+        assert _target() is sentinel
diff --git a/betty/typing.py b/betty/typing.py
index f0e56ef51..e4a0eeb7f 100644
--- a/betty/typing.py
+++ b/betty/typing.py
@@ -2,19 +2,46 @@
 Providing typing utilities.
 """
 
+import re
 from typing import TypeVar
 
 
 _T = TypeVar("_T")
 
 
+_INTERNAL_INDENTATION_PATTERN = re.compile(r"( +)")
+
+
+def _internal(target: _T) -> _T:
+    doc = target.__doc__ or ""
+    doc_last_line = doc.rsplit("\n")[-1]
+    indentation_match = _INTERNAL_INDENTATION_PATTERN.match(doc_last_line)
+    indentation = indentation_match.group(0) if indentation_match else ""
+    target.__doc__ = (
+        doc
+        + f"\n\n{indentation}This is internal. It **MAY** be used anywhere in Betty's source code, but **MUST NOT** be used by third-party code."
+    )
+    return target
+
+
+@_internal
 def internal(target: _T) -> _T:
     """
     Mark a target as internal to Betty.
 
     Anything decorated with ``@internal`` MAY be used anywhere in Betty's source code,
     but MUST be considered private by third-party code.
+    """
+    return _internal(target)
+
+
+@internal
+def public(target: _T) -> _T:
+    """
+    Mark a target as publicly usable.
 
-    This function is internal (and ironically cannot be decorated with itself).
+    This is intended for items nested inside something marked with :py:func:`betty.typing.internal`,
+    such as class attributes: third-party code **SHOULD NOT** use a class marked ``@internal``
+    directly, but **MAY** use any of its attributes that are marked ``@public``.
     """
     return target