diff --git a/doc/source/guidelines.rst b/doc/source/guidelines.rst
index 484af7e..e610f53 100644
--- a/doc/source/guidelines.rst
+++ b/doc/source/guidelines.rst
@@ -45,6 +45,9 @@ And recommend the following code to use plural form::
 The contextual form and plural form are used only when needed.
 By default, the translation should use the ``_()``.
 
+.. note::
+   These two functions were only available in oslo.i18n >= 2.1.0.
+
 Log Translation
 ===============
 
diff --git a/doc/source/usage.rst b/doc/source/usage.rst
index fb8c0ce..14c0b35 100644
--- a/doc/source/usage.rst
+++ b/doc/source/usage.rst
@@ -41,9 +41,11 @@ the marker functions the factory creates.
     _ = _translators.primary
 
     # The contextual translation function using the name "_C"
+    # requires oslo.i18n >=2.1.0
     _C = _translators.contextual_form
 
     # The plural translation function using the name "_P"
+    # requires oslo.i18n >=2.1.0
     _P = _translators.plural_form
 
     # Translators for log levels.