Kunai is a small tool used by ninja. Kunai has a shape which can be easily hidden. Kunai is written as '苦内' or '苦無' in Chinese characters.
Kunai was originally a tool which was treated as an utility knife or an all-purpose scoop in the general market at the time and thus ninja began to use it as a weapon because no one was suspicious of a person who carried a kunai with him.
Two variations are dai-kunai (big kunai) (ranging about from 13 to 15 cm in length) and sho-kunai (small kunai) (ranging about from 8 to 10 cm in length).
Kunai has flat claws made of iron and it was used to climb walls or dig holes in walls or ground.
Kunai is well known as well as shuriken (ninja throwing stars, multi-pointed throwing weapon) and makibishi (small, spiked caltrops used by ninja) as tools used by ninja, but kunai was little used except the case where it was needed immediately as a substitution because it was made of a high quality wrought iron.
Small kunai may be used like shuriken and it is called 'tobi-kunai'.
Kunai is made by forging wakugi (Japanese nail) by hand and forming into the shape of kunai. Kunai is not a standard article and therefore it comes in various sizes and shapes, and its center of gravity is unstable because it was originally made as a tool, not as a throwing weapon, as described above. Therefore it was said that only a very skilled person could hit a target because stabbing the target with a kunai was more difficult than with a shuriken.