What is Cream of tartar?

Cream of tartar (potassium hydrogen tartrate) is a component of baking powder. (Baking powder comprises baking soda and cream of tartar.) Cream of tartar is a by-product of winemaking; it's derived from refined tartaric acid, which forms on the inside of wine barrels, or from the whitish crystals (known as 'white diamonds') that precipitate out of some wines.

It's used in baking and in making desserts. It gives a creamy texture to icings and is used to stabilise and increase the volume of beaten egg whites, so is often used in making meringues.