diff --git a/Include/object.h b/Include/object.h
index dfeb43bda7d841..85abd30b5ad7d6 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -265,6 +265,7 @@ _Py_ThreadId(void)
     #if defined(__clang__) && _Py__has_builtin(__builtin_thread_pointer)
     tid = (uintptr_t)__builtin_thread_pointer();
     #else
+    // r13 is reserved for use as system thread ID by the Power 64-bit ABI.
     register uintptr_t tp __asm__ ("r13");
     __asm__("" : "=r" (tp));
     tid = tp;
@@ -273,6 +274,7 @@ _Py_ThreadId(void)
     #if defined(__clang__) && _Py__has_builtin(__builtin_thread_pointer)
     tid = (uintptr_t)__builtin_thread_pointer();
     #else
+    // r2 is reserved for use as system thread ID by the Power 32-bit ABI.
     register uintptr_t tp __asm__ ("r2");
     __asm__ ("" : "=r" (tp));
     tid = tp;