A tachi is a curved sword with a blade length normally longer than 23.6" (60cm). These blades were made during the Koto times and were worn with the edge down and suspended from two cords. This is different than Katana which had the sword worn on the belt and edge up. At the end of the Koto period many of the tachi were shortened to katana length. To shorten a blade the nakago or tang end is shortened. Many times shortened swords  have the tip of the nakago , called nakagojiri, squared off. This squared off shape is called kirijuri.