diff --git a/ext/readline/extconf.rb b/ext/readline/extconf.rb
index 5e3c9ea982..70dfd0b6c2 100644
--- a/ext/readline/extconf.rb
+++ b/ext/readline/extconf.rb
@@ -99,6 +99,9 @@ readline.have_func("rl_redisplay")
 readline.have_func("rl_insert_text")
 readline.have_func("rl_delete_text")
 unless readline.have_type("rl_hook_func_t")
+  # rl_hook_func_t is available since readline-4.2 (2001).
+  # Function is removed at readline-6.3 (2014).
+  # However, editline (NetBSD 6.1.3, 2014) doesn't have rl_hook_func_t.
   $DEFS << "-Drl_hook_func_t=Function"
 end