What the Shirasaya Is:

The shirasaya is a "resting saya" or "resting scabbard" and is not intended or designed for active use. One might think of the shirasaya as a warm, cozy sleeping bag for the sword's blade.

Back when when the Samurai wore his katana daily, there would inevitably be occasions when the blade required storage for a period of time, such as when the blade was first forged then polished and was subsequently waiting for the koshirae ("working mounts" as it were) to be completed, or when transporting the blade to and from various craftsmen for repairs, polishing, remounting and the like. Shirasaya were also commonly used when a blade was eventually retired or when the samurai himself might retire from active service thus storing the blade for future generations.

Under such circumstances, the sword would typically be un-mounted from the Koshirae and mounted in the shirasaya which is designed specifically for preservation and protection. Even today, a high-end katana blade is often forged, polished and mounted in shirasaya before shipping to the customer who will then determine what craftsman he wants to do the work on the koshirae.

What the Shirasaya Is Not:

The Shirasaya is not a working mount and should never be considered as such, nor is it simply a very plain, unadorned koshirae. The shirasaya is neither intended nor designed for cutting or kata of any kind, is ill suited to that intention and can not typically deal with the stresses experienced in such use.

Likewise, the shirasaya is not a way to hide the sword or to disguise it in the fashion of a sword cane.

There is often some confusion amongst newcomers between the shirasaya and the shikomi-zue. Therefore, by way of clarification, we offer the following.

Shikomi-zue was a way of mounting the blade so as to hide it in what looks like a walking stick, cane, or other similar yet innocuous item. The reasons were diverse, of course, but in the Meiji era this became popular because of the ban on Samurai wearing swords openly as well as the decree which closely followed, that ordered all katana be surrendered to the Imperial Authorities.

Even during the Meiji era, the shikomi-zue was not as prevalent or as widespread as is sometimes believed. The image of swarms of clandestine Samurai and/or Ninja skulking about town ready to unleash their stealthy swords at a second's provocation is largely the fanciful notion of movie makers.

Certainly, there was a clandestine motive behind the shikomi-zue (hiding illegal weapons from the authorities) and yes, there were those who saw it as an advantageous assassin's ploy, but for the most part, the use of shikomi-zue as an assassin's sword is a very handy plot device.