diff --git a/include/pqrs/hid/usage.hpp b/include/pqrs/hid/usage.hpp
index 40b3438..0e9f065 100644
--- a/include/pqrs/hid/usage.hpp
+++ b/include/pqrs/hid/usage.hpp
@@ -346,6 +346,9 @@ constexpr value_t volume_decrement(0x00ea);
 
 // application launch buttons
 
+// AL Consumer Control Configuration (0x0183) is used as open music player.
+// - https://github.com/pqrs-org/cpp-hid/pull/1
+// - https://source.android.com/docs/core/interaction/input/keyboard-devices
 constexpr value_t al_music_player(0x0183);
 constexpr value_t al_word_processor(0x0184);
 constexpr value_t al_text_editor(0x0185);