diff --git a/Help/prop_tgt/INTERFACE_SYSTEM_INCLUDE_DIRECTORIES.rst b/Help/prop_tgt/INTERFACE_SYSTEM_INCLUDE_DIRECTORIES.rst
index b54b6c1d8..a0a97ada7 100644
--- a/Help/prop_tgt/INTERFACE_SYSTEM_INCLUDE_DIRECTORIES.rst
+++ b/Help/prop_tgt/INTERFACE_SYSTEM_INCLUDE_DIRECTORIES.rst
@@ -7,8 +7,15 @@ Targets may populate this property to publish the include directories
 which contain system headers, and therefore should not result in
 compiler warnings.  The :command:`target_include_directories(SYSTEM)`
 command signature populates this property with values given to the
-``PUBLIC`` and ``INTERFACE`` keywords.  Projects may also get and set the
-property directly.
+``PUBLIC`` and ``INTERFACE`` keywords.
+
+Projects may also get and set the property directly, but must be aware that
+adding directories to this property does not make those directories used
+during compilation.  Adding directories to this property marks directories
+as ``SYSTEM`` which otherwise would be used in a non-``SYSTEM`` manner.  This
+can appear similar to 'duplication', so prefer the
+high-level :command:`target_include_directories(SYSTEM)` command and avoid
+setting the property by low-level means.
 
 When target dependencies are specified using :command:`target_link_libraries`,
 CMake will read this property from all target dependencies to mark the