The first use of katana as a word to describe a long sword that was different from a tachi occurs as early as the Kamakura Period (1185–1333). These references to "uchigatana" and "tsubagatana" seem to indicate a different style of sword, possibly a less costly sword for lower-ranking warriors. The Mongol invasions of Japan facilitated a change in the designs of Japanese swords. Thin tachi and chokutō-style blades were often unable to cut through the boiled leather armour of the Mongols, with the blades often chipping or breaking off. The evolution of the tachi into what would become the katana seems to have continued during the early Muromachi period (1337 to 1573). Starting around the year 1400, long swords signed with the katana-style mei were made. This was in response to samurai wearing their tachi in what is now called "katana style" (cutting edge up). Japanese swords are traditionally worn with the mei facing away from the wearer. When a tachi was worn in the style of a katana, with the cutting edge up, the tachi's signature would be facing the wrong way. The fact that swordsmiths started signing swords with a katana signature shows that some samurai of that time period had started wearing their swords in a different manner.
The rise in popularity of katana amongst samurai came about due to the changing nature of close-combat warfare. The quicker draw of the sword was well suited to combat where victory depended heavily on short response times. The katana further facilitated this by being worn thrust through a belt-like sash (obi) with the sharpened edge facing up. Ideally, samurai could draw the sword and strike the enemy in a single motion. Previously, the curved tachi had been worn with the edge of the blade facing down and suspended from a belt.
The length of the katana blade varied considerably during the course of its history. In the late 14th and early 15th centuries, katana blades tended to have lengths between 70 and 73 centimetres (27 1⁄2 and 28 3⁄4 in). During the early 16th century, the average length dropped about 10 centimetres (4 in), approaching closer to 60 centimetres (23 1⁄2 in). By the late 16th century, the average length had increased again by about 13 centimetres (5 in), returning to approximately 73 centimetres (28 3⁄4 in).
The katana was often paired with a smaller companion sword, such as a wakizashi, or it could also be worn with a tantō, a smaller, similarly shaped dagger. The pairing of a katana with a smaller sword is called the daishō. Only samurai could wear the daishō: it represented their social power and personal honour.