diff --git a/src/os/win32/ngx_files.h b/src/os/win32/ngx_files.h
index 5aef0b6ef..132b6221b 100644
--- a/src/os/win32/ngx_files.h
+++ b/src/os/win32/ngx_files.h
@@ -27,6 +27,9 @@
 #define NGX_FILE_ERROR              0
 
 
+/*
+ * FILE_FLAG_BACKUP_SEMANTICS allows to obtain a handle to a directory
+ */
 
 #define ngx_open_file(name, mode, create, access)                            \
     CreateFile((const char *) name, mode,                                    \