The wakizashi has a blade between 30 and 60 cm (12 and 24 in), with wakizashi close to the length of a katana being called o-wakizashi and wakizashi closer to tantō length being called ko-wakizashi. The wakizashi being worn together with the katana was the official sign that the wearer was a samurai or swordsman of feudal Japan. When worn together the pair of swords were called daishō, which translates literally as "big-little". The katana was the big or long sword and the wakizashi the companion sword.Wakizashi are not necessarily just a smaller version of the katana; they could be forged differently and have a different cross section.
Wakizashi have been in use as far back as the 15th or 16th century. The wakizashi was used as a backup or auxiliary sword; it was also used for close quarters fighting, to behead a defeated opponent and sometimes to commit ritual suicide. The wakizashi was one of several short swords available for use by samurai including the yoroi tōshi, the chisa-katana and the tantō. The term wakizashi did not originally specify swords of any official blade length and was an abbreviation of "wakizashi no katana" ("sword thrust at one's side"); the term was applied to companion swords of all sizes. It was not until the Edo period in 1638 when the rulers of Japan tried to regulate the types of swords and the social groups which were allowed to wear them that the lengths of katana and wakizashi were officially set.
Kanzan Satō, in his book titled "The Japanese Sword", notes that there did not seem to be any particular need for the wakizashi and suggests that the wakizashi may have become more popular than the tanto due to the wakizashi being more suited for indoor fighting. He mentions the custom of leaving the katana at the door of a castle or palace when entering while continuing to wear the wakizashi inside. While the wearing of katana was limited to the samurai class, wakizashi of legal length (ko-wakizashi) could be carried by the chonin class which included merchants. This was common when traveling due to the risk of encountering bandits.Wakizashi were worn on the left side, secured to the waist sash (Uwa-obi or himo).
^A glossary of the construction, decoration and use of arms and armor in all countries and in all times, together with some closely related subjects, George Cameron Stone, Jack Brussel Pub., 1961 P.201