diff --git a/include/boost/system/api_config.hpp b/include/boost/system/api_config.hpp
index adbe355..28b8bec 100644
--- a/include/boost/system/api_config.hpp
+++ b/include/boost/system/api_config.hpp
@@ -1,24 +1,42 @@
-#ifndef BOOST_SYSTEM_API_CONFIG_HPP_INCLUDED
-#define BOOST_SYSTEM_API_CONFIG_HPP_INCLUDED
+//  boost/system/api_config.hpp  -------------------------------------------------------//
 
-// Copyright Beman Dawes 2003, 2006, 2010
-// Copyright 2018 Peter Dimov
+//  Copyright Beman Dawes 2003, 2006, 2010
+
+//  Distributed under the Boost Software License, Version 1.0.
+//  See http://www.boost.org/LICENSE_1_0.txt
+
+//  See http://www.boost.org/libs/system for documentation.
+
+//--------------------------------------------------------------------------------------//
+
+//  Boost.System calls operating system API functions to implement system error category
+//  functions. Usually there is no question as to which API is to be used.
 //
-// Distributed under the Boost Software License, Version 1.0.
-// See http://www.boost.org/LICENSE_1_0.txt
+//  In the case of MinGW or Cygwin/MinGW, however, both POSIX and Windows API's are
+//  available. Chaos ensues if other code thinks one is in use when Boost.System was
+//  actually built with the other. This header centralizes the API choice and prevents
+//  user definition of API macros, thus elminating the possibility of mismatches and the
+//  need to test configurations with little or no practical value.
 //
-// See http://www.boost.org/libs/system for documentation.
 
-// This header is no longer used by Boost.System.
-// Its use is discouraged and it will be deprecated.
+//--------------------------------------------------------------------------------------//
 
-// Definition of BOOST_*_API retained for compatibility.
+#ifndef BOOST_SYSTEM_API_CONFIG_HPP                  
+#define BOOST_SYSTEM_API_CONFIG_HPP
 
-#if defined(_WIN32) || defined(__CYGWIN__)
-// Windows default, including MinGW and Cygwin
-#  define BOOST_WINDOWS_API
+# if defined(BOOST_POSIX_API) || defined(BOOST_WINDOWS_API)
+#   error user defined BOOST_POSIX_API or BOOST_WINDOWS_API not supported
+# endif
+
+//  BOOST_POSIX_API or BOOST_WINDOWS_API specify which API to use
+//    Cygwin/MinGW does not predefine _WIN32.
+//    Standalone MinGW and all other known Windows compilers do predefine _WIN32
+//    Compilers that predefine _WIN32 or __MINGW32__ do so for Windows 64-bit builds too.
+
+# if defined(_WIN32) || defined(__CYGWIN__) // Windows default, including MinGW and Cygwin
+#   define BOOST_WINDOWS_API
 # else
-#  define BOOST_POSIX_API
-#endif
+#   define BOOST_POSIX_API 
+# endif
                                      
-#endif  // BOOST_SYSTEM_API_CONFIG_HPP_INCLUDED
+#endif  // BOOST_SYSTEM_API_CONFIG_HPP