Kogatana

The kogatana was an accessory knife associated with the wakizashi (which originally meant 'side arm). Samurai would wear this shorter sword along with their katana as a pair and this would be known as the daishō (a contraction of daitō, long sword, and shōtō, small sword, thus meaning big-small)

References of wakizashi being used date back to the 16th Century and Turnbull in his book Warriors of Medieval Japan implies that the wakizashi was used during the Age of Warring States or the Sengoku period of mid 15th Century to 17th Century Japan.

A small knife was often carried in a small pouch on the wakizashi and was known as the kogatana after the blade or the kozuka after the hilt. 'Weapon-A Visual History of Arms and Armoury' (DK Books) maintains that this small blade, while very sharp, would be used as an accessory knife for cutting paper and envelopes perhaps. Bugei.com states the kozuka, "have many uses including being used as a small throwing weapon".

check out some examples of the kogatana here:

http://www.ncjsc.org/gloss_kogatana.htm

Below is a film about sword forging and in this example they are making the short blade or kogatana.

Interestingly medieval European hunting swords may also use scabbards with 'trousses' within which would be held small knives for meat trimming or a bodkin for piercing holes.