The wakizashi (translates to "side inserted sword" or "medium-length sword" depending where you look), was a companion blade for a larger sword, such as a katana or a tachi. During the Edo period, it was codified into law that a samurai must wear both his katana and a shorter sword to signify his status, which then formed the daishō (large and small). Generally, the wakizashi was chosen, though this could be done with a tanto as well. During this period commoners were also permitted to carry a single wakizashi for self-defence as long as it was of legal length, though what was considered "legal length" doesn't seem to have ever properly been codified. 

The wakizashi dates back to the 15th or 16th century and could vary in length, with blades being anywhere between 1 and 2 shaku (12"-24"). They were not necessarily simply a smaller version of the katana or tachi, as their forging could vary significantly. It is believed that their primary uses were as a back-up weapon, for close quarters and indoor combat, and ritualistic purposes. They could also be worn when a samurai was a guest in another's home or castle, while the katana had to be left at the door. 

This wakizashi by Dragon King is a modern take on the classic companion blade, featuring a blade of nearly 20" inches of differentially tempered 1566 steel with a traditional hamon line, it uses modern G-10 laminate for the tsuka (the grip) and on each side it features an inset shi shi (lion dog) for additional decoration and a better gripping surface. 

Specs:
Overall Length: 27"
Handle Length: 6 1/4"
Blade Length: 19 1/2"
Blade Steel: 1566
Blade Thickness: 6mm-3mm"
Weight: 1lb 7oz