The earliest know japanese swords were straight and thought to have come frm china. The tachi was a curved sword and just a little longer than the katana. The katana is kind of like the little brother of the tachi. The main differences in the tachi and the katana is the length, how it is carried, the fittings and the signature. Any blad over 28"s is concidered a tachi. The tachi is carried hanging with the blade down, katana are carried n the belt blade up. Tha tachi has a slightly curved handle and dfferent montings, the katana has a straight handle. Since the smiths signature is always facing out from the person when the sword is worn, the signature on the tachi would be on the side with the blade edge facing to the right, the katana would have the signature on the side of the blade with the edge facing to the left. The tachi also had a bit more of a curve than the katana.