Potassium sodium tartrate is a double salt

Double salt

Double salts are salts containing more than one cation or anion. They form when more than one salt is dissolved in a liquid and when together they crystallize in a regular pattern. A well-known double salt is alum containing two cations and a sulfate anion...

 first prepared (in about 1675) by an apothecary

Pharmacy

Pharmacy is the health profession that links the health sciences with the chemical sciences and it is charged with ensuring the safe and effective use of pharmaceutical drugs...

, Pierre Seignette, of La Rochelle

La Rochelle

La Rochelle is a city in western France and a seaport on the Bay of Biscay, a part of the Atlantic Ocean. It is the capital of the Charente-Maritime department.The city is connected to the Île de Ré by a bridge completed on 19 May 1988...

, France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

. As a result the salt was known as Seignette's salt or Rochelle salt.
Rochelle salt is not to be confused with rock salt

Halite

Halite , commonly known as rock salt, is the mineral form of sodium chloride . Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities...

, which is simply the mineral form of sodium chloride.
Potassium sodium tartrate and monopotassium phosphate

Monopotassium phosphate

Monopotassium phosphate -- 24 -- is a soluble salt which is used as a fertilizer, a food additive and a fungicide. It is a source of phosphorus and potassium. It is also a buffering agent...

 were the first materials discovered to exhibit piezoelectricity

Piezoelectricity

Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...

. This property led to its extensive use in "crystal" gramophone (phono) pick-ups, microphones and earpieces during the post-War consumer electronics boom of the mid-20th Century.