wakizashi

A wakizashi is a Japanese sword, longer than a tanto and shorter than a katana. They have a blade length of 18 inches, on average. It's also known as a kodachi. In the early 1600's, when Miyamoto Musashi popularized carrying and weilding two swords at once, samurai began pairing their katana with a wakizashi rather than a tanto, since a wakizashi is much more effective in combat. The custom of wearing a wakizashi along side a katana (called daisho) continued for centuries until carrying weapons in public was made illegal.

It was worn by the samurai, during the Momoyama and Edo period along with the katana as part of a matching set called a daisho. If the katana was broken, the shorter wakizashi could be used in its place. It was also considered 'bad etiquette' to wear a long sword indoors, so by the use of a wakizashi the Samurai would not be vulnerable indoors.

The use of the wakizashi did not however stop with the samurai. Often they were manufactured for 'non-warrior class' individuals such as merchants or artisans who were forbidden to wear long swords. For this reason it became a duel purpose weapon, for use both indoors and by anyone who could afford it.

Due to the different markets for this short sword it would be made in one of two ways. Firstly simple and elegant as part of a daisho for the purist warrior, secondly more ornate with vibrant carvings for the more refined tastes of the middle class.

As with all swords, it was often made with a specific person in mind and so length would often be dictated by height. If a wakizashi were made for a tall man it would be referred to as an o-wakizashi due to the extra length. Transversely a shorter blade was referred to as a ko-wakizashi.

References
http://swordforum.com/
http://www.coling.uni-freiburg.de/~neuhaus/iaido/manual/node5.html