Tachi
Authentic tachi were forged during the Kotō period, before 1596. With a few exceptions katana and tachi can be distinguished from each other if signed, by the location of the signature (mei) on the tang (nakago). In general themei should be carved into the side of the nakago that would face outward when the sword was worn. Since a tachi was worn cutting edge down, and the katana was worn cutting edge up the mei would be in opposite locations on the nakago of both types of swords.
An authentic tachi that was manufactured in the correct time period averaged 70–80 centimeters (27 9/16 – 31 1/2 inches)in cutting edge length (nagasa) and compared to a katana was generally lighter in weight in proportion to its length, had a greater taper from hilt to point, was more curved with a smaller point area.
Unlike the traditional manner of wearing the katana, the tachi was worn hung from the belt with the cutting-edge down, and was most effective when used by cavalry.Deviations from the average length of tachi have the prefixesko- for “short” and ō- for “great or large” attached. For instance, tachi that were shōtō and closer in size to a wakizashi were called kodachi. The longest tachi (considered a 15th century ōdachi) in existence is more than 3.7 meters in total length (2.2m blade) but believed to be ceremonial. In the late 1500s and early 1600s many old surviving tachi blades were converted into katana by having their original tangs cut (o-suriage), the signature (mei) would be lost in this process.
Hira Zukuri
Basically a bevel which slopes from the cutting edge of the blade all the way to the back. Rarely seen on full length swords, usually used when making tanto and wakizashi. It forms a lighter and much sharper blade with greatly enhanced cutting ability at the cost of durability. You wouldn’t want to go up against a sword wielding opponent with one of these if you find yourself having to block and parry often. If however one wanted a very light weight but very effective offensive-only sword this is the way to go. In the world of competitive tameshigiri cutting these are often referred to as ‘cheater blades’ due to their cutting effectiveness.
Here is an example of a Hira Zukuri style blade. Note the complete lack of a flat deflecting side edge.
David P. Roberts