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 the mei 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 thekatana 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 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 prefixes ko- for “short” and ō- for “great or large” attached. For instance, tachi that were shōtōand 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.
For a sword to be worn in “tachi style” it needed to be mounted in a tachi koshirae. The tachi koshirae had two hangers (ashi) which allowed the tachi to be worn in a horizontal position with the cutting edge down. A sword not mounted in a tachi koshirae could be worn tachi style by use of a koshiate, a leather device which would allow any sword to be worn in the tachi style.