diff --git a/src/bin/topola/cli.rs b/src/bin/topola/cli.rs
index 3a28f7d..d84a745 100644
--- a/src/bin/topola/cli.rs
+++ b/src/bin/topola/cli.rs
@@ -9,7 +9,7 @@ pub struct Cli {
 	  help = "Specify the Specctra Design (*.dsn) input file for the Topola autorouter")]
     pub input: PathBuf,
     #[arg(short, long, value_name = "SESSION FILE",
-	  help = "Specify the output Topola session file (*.ses)")
+	  help = "Specify the output Topola session file (*.ses), by default, it is the input filename with extension changed to .ses")
     ]
     pub output: Option<PathBuf>,
     #[arg(short, long, value_name = "FILE", help = "TODO")]