Yari is the Japanese term for spear, or more specifically, the straight-headed spear. The samuraii trained in wielding the yari, called sōjutsu. Yari measured anywhere from one meter to upwards of six meters (3.3 to 20 feet). The longer versions were called ōmi no yari while shorter ones were known as mochi yari or tae yari.
The common height of the yari varies by wielder:
- The samurai usually used yari measuring one to three meters on average (3.3 to 9.9 feet).
- The ashigaru usually used yari measuring anywhere from four to six meters(13.2 to 20 feet).
The yari was characterized by a straight blade at the end of the shaft. The blade measured anywhere from several centimeters long, to 3 feet (0.9 meters) or more. The blades were made out of the same high-quality steel as the katana and japanese arrowheads forged by samurai.
Su yari (素槍, simple spear) Edit
The sankaku yari (三角槍, triangle spear) had a point that resembled a narrow spike with a triangular cross-section. A sankaku yari therefore had no cutting edge, only a sharp point at the end. The sankaku yari was therefore best suited for penetrating armor, even armor made of metal, which a standard yari was not as suited to.
The fukuro yari (袋槍, bag spear or socket spear) sported a more European style fitting of the straight head. Instead of the yari's traditional very long embedded tang, an entirely metal socket which slipped over the narrowed end of the pole, The unit was forged as a single piece of both socket and blade. This design was rare next to the traditional 'long-tang' configuration.
A kuda yari (管槍, tube spear) was not very different in construction than another simple choku yari. However for this spear, the upper hand gripped a hollow metal tube that allowed the yari to "screw" while being thrust. This style of sojutsu is typified in the school Owari Kan Ryū.
Kikuchi yari (菊池槍, spear of Kikuchi) were one of the rarest designs, possessing only a single edge. This created a weapon that could be used for hacking and almost resembled a straight edged naginata.
Yajiri nari yari (鏃形槍, spade-shaped spear) had a very broad "spade-shaped" head. It often had a pair of holes centering the two ovoid halves.
Kama yari (鎌槍, sickled spear) Edit
These spears were very effective weapons though their more complex blade shapes were extremely difficult to properly forge and sharpen; therefore these were far less common than the above types and were often used for ornamental purposes.
Magari yari (曲槍, curved spear), also called jūmonji yari (十文字槍, cross-shaped spear), looked something similar to a trident or partisan and brandished a pair of curved blades around its central lance. Occasionally called maga yari in modern weaponry texts. The 
kama yari (鎌槍, sickle spear) gets its name from a peasant weapon called kama (lit. sickle or scythe). However, a kama isn't a scythe as most Westerners think of it, a giant, curved blade connected at right angles to a two-meter-long wooden handle, but rather a much smaller version, with a less dramatically curved blade and a straight wooden handle approximately two feet long.
The kata kama yari (片鎌槍, single-sided sickle spear) had a radical weapon design sporting a blade that was two-pronged. Instead of being constructed like a military fork, a straight blade (as in su yari) was intersected just below its midsection by a perpendicular blade. This blade was slightly shorter than the primary, had curved tips making a parallelogram, and was set off center so that only 1/6th of its length extended on the other side. This formed a kind of messy 'L' shape.
The tsuki nari yari (月形槍, moon-shaped spear) barely looked like a 'spear' at all. A polearm that had a crescent blade for a head, this could be used for slashing and hooking.
A kagi yari (鉤槍, hook spear) had a long blade with a side hook much like that found on a fauchard. This could be used to catch another weapon, or even dismount a rider on horseback.
Bishamon yari possessed some of the most ornate designs for any spear. Running parallel to the long central blade were two 'crescent moon' shaped blades facing outwards. They were attached in two locations by short cross bars, making the head look somewhat like a fleur-de-lis.
The yari was started being used by samurai in the early 1200s where the yari derived from the chinese spear ji. The original warfare for bushi was not for the commoners of Japan, where the bushi would change eachother to duals via horseback archery and sword . That was until the mongol invasion in 1274 and 1281 where the mongols used the chinese-employed mongol footmen wielding long pikes to fight in tight formation, and move in large units to stave off cavalry.Polearms (including naginata and yari) were of much greater military use than swords, due to their much greater range, their lesser weight per unit length (though overall a polearm would be fairly hefty), and their great piercing ability. Swords in a full battle situation were therefore relegated to emergency sidearm status from the Heian through the Muromachi periods.