Isn't that a strange name for a musical instrument? Nevertheless plate is a flat metal plate is one of the most common percussion instruments. In the orchestra plates appeared in the XVII century, and were known for many centuries before. On the plates played 4 thousand years ago in Ancient Egypt.
The orchestra is usually a pair of plates (so as timpani, they are called in the plural). The sound is extracted kick plates together, blow on the plate stick on the drum or timpani, hit a metal whisk. The sound of plates, depending on the type and severity of the impact can be very diverse, from ringing, piercing, until barely audible whisper. Often the shock plates coincide with the sound of a big drum. It happens that one of the plates screwed to the drum, and plays both instruments one performer.
Plates - tool with indefinite pitch. Party them is written on a single line - "the thread".