diff --git a/stl/inc/__msvc_threads_core.hpp b/stl/inc/__msvc_threads_core.hpp
index 4afe27a5a2a..3bb43713a27 100644
--- a/stl/inc/__msvc_threads_core.hpp
+++ b/stl/inc/__msvc_threads_core.hpp
@@ -33,13 +33,12 @@ struct _Stl_critical_section {
 };
 
 struct _Mtx_internal_imp_t {
-#if defined(_CRT_WINDOWS) || defined(UNDOCKED_WINDOWS_UCRT) // for Windows-internal code
-    static constexpr size_t _Critical_section_size = 2 * sizeof(void*);
-#elif defined(_WIN64) // ordinary 64-bit code
+// TRANSITION, ABI: We should directly store _M_srw_lock above.
+#ifdef _WIN64
     static constexpr size_t _Critical_section_size = 64;
-#else // vvv ordinary 32-bit code vvv
+#else // ^^^ 64-bit / 32-bit vvv
     static constexpr size_t _Critical_section_size = 36;
-#endif // ^^^ ordinary 32-bit code ^^^
+#endif // ^^^ 32-bit ^^^
 
     int _Type{};
     union {
@@ -60,13 +59,12 @@ struct _Stl_condition_variable {
 #pragma warning(push)
 #pragma warning(disable : 26495) // Variable 'meow' is uninitialized. Always initialize a member variable (type.6).
 struct _Cnd_internal_imp_t {
-#if defined(_CRT_WINDOWS) // for Windows-internal code
-    static constexpr size_t _Cnd_internal_imp_size = 2 * sizeof(void*);
-#elif defined(_WIN64) // ordinary 64-bit code
+// TRANSITION, ABI: We should directly store _Win_cv above.
+#ifdef _WIN64
     static constexpr size_t _Cnd_internal_imp_size = 72;
-#else // vvv ordinary 32-bit code vvv
+#else // ^^^ 64-bit / 32-bit vvv
     static constexpr size_t _Cnd_internal_imp_size = 40;
-#endif // ^^^ ordinary 32-bit code ^^^
+#endif // ^^^ 32-bit ^^^
 
     union {
         _Stl_condition_variable _Stl_cv{};