We usually associate gongs with China. Actually, a gong is an East and Southeast Asian musical instrument in the form of a flat metal disc that is hit with a mallet to make a sound. There are three types of this instrument: suspended, bossed, and bowl.
The suspended type is a rather flat, circular metal disc hung vertically with a cord through holes near the rims top part. The bossed type has a raised center boss, usually hung and played horizontally. The bowl type is in the shape of a bowl, placed on cushions.
The bowl type is less a gong and more of a bell. Bronze and brass are usual material used to make this instrument. Other alloys are sometimes used to make this instrument.
The suspended type has beaters to make it sound. It has two kinds: flat faced disc with or without a turned edge, and that with a raised center boss. The bigger the gongs, the bigger and softer are their beaters.
In Western symphonic music, a flat-faced gong is typically called a tam-tam to differentiate it from its bossed counterpart, although calling it gong is all right for either type. The gong has been used in China since ancient times. Its original purpose was to signal peasants working in the fields.
These instruments can be so loud their sound can reach up to 50 miles. Traditionally, in Japan, they are used to signal the start of a sumo-wrestling competition. A huge flat gong can be primed by hitting it lightly before the main stroke is delivered.
The trick of keeping the priming stroke muffled takes a lot of skill. The tiniest suspended types can be played with either bamboo sticks or drumsticks. The most well-known types to Westerners are the chau (bullseye) gongs.