A katar is a unique type of knife which was once widely used on the Indian subcontinent. It is often associated specifically with Hindu culture in India, although people of other backgrounds probably used katars as well, since they are extremely versatile knives which come in a myriad of configurations to meet various needs. Katars are not in widespread use anymore, although many museums have very fine examples of antique katars on display, for people who want to get a closer look at these famous knives.
The defining feature of a katar is the handle, which consists of two parallel bars supported by one or more crosspieces. To hold the knife, people wrap a hand around the crosspiece, causing the blade to protrude above their knuckles, almost like a talon, while the bars run down the user's fist and arm. To use the knife, it is necessary to punch, rather than stab, putting the full force of the body behind the knife and using it like a natural extension of the hand.
Katar blades are quite varied. They are classically double edged, and vary in length. Some katars have forked blades, in which case there may be two or three blades mounted on the handle, and in some regions the blades are movable, allowing people to penetrate a victim with a single blade and then trigger a mechanism which causes the blade to split, causing formidable internal injuries. Some 19th century katars even included small guns.
Most katars were designed for close range use, and they could be highly effective against armor of various types. The punching motion required to use the blade pushed users to develop styles of fighting which resemble many martial arts, with a wide range of moves and styles of punches which could be paired with a katar for maximum effect. Katars were also sometimes used in ritual or religious settings, which may explain why they have become so closely associated with Hindu culture.
The katar is sometimes called an Indian or Bundi dagger, and in some cases the blades were long enough to qualify the knife as a short sword. Some people still train with weapons which resemble the classic katar, and several companies continue to manufacture katars for hobbyists and for film use. If you decide to purchase a katar for your personal weapons collection, you may want to think about whether you want an antique or modern blade, and if you aren't familiar with metals, bring a metalsmith along on your buying expedition, as many katars are made with cheap metals which corrode and do not hold an edge.