The Reinheitsgebot literally "purity order"), sometimes called the "German Beer Purity Law" in English, is the collective name for a series of regulations limiting the ingredients in beer in Germany and its predecessor states. The most well-known version of the law was adopted in Bavaria in 1516, but similar regulations predate the Bavarian order, and modern regulations also significantly differ from the 1516 Bavarian version.
The most influential predecessor of the modern Reinheitsgebot was a law first adopted in the duchy of Munich in 1487. After Bavaria was reunited, the Munich law was adopted across the entirety of Bavaria on April 23, 1516. As Germany unified, Bavaria pushed for adoption of this law on a national basis (see Broader adoption).
German beer: 500 years of 'Reinheitsgebot' rules
This weekend marks 500 years since the Duke of Bavaria introduced the "Reinheitsgebot" or purity law - strict rules controlling what can go into beer.
And beer lovers across Germany will be celebrating at events (read: subsidised drinking opportunities) to mark the anniversary of the famous food law.
As Chancellor Angela Merkel partakes of an obligatory Pils with German brewers in the Bavarian town of Ingolstadt, the BBC's Claudia Allen takes a look at the Reinheitsgebot, and what it means for German beer today.
Why was the purity law introduced?
The decree known as the Reinheitsgebot, issued in Ingolstadt in 1516, had three aims: to protect drinkers from high prices; to ban the use of wheat in beer so more bread could be made; and to stop unscrupulous brewers from adding dubious toxic and even hallucinogenic ingredients as preservatives or flavourings.
They included herbs and spices such as rosemary and caraway, henbane, thorn-apple, wood shavings, roots, soot or even pitch, according to the German Brewers' Association (DBB).
Duke Wilhelm IV's beer purity regulation of 1516, which was preceded by earlier rules on beer production, was gradually implemented in other parts of southern Germany. It eventually became law in the north and thus the whole country in 1906.
The DBB claims that the Reinheitsgebot is the oldest currently valid consumer protection law in the world.
What can go into German beer?
The original law limited ingredients to just barley, hops and water.
The exact role of yeast in alcoholic fermentation was not understood at the time and it was only later that brewers were able to add the micro-organism as a specific ingredient.
The production of wheat beers remained limited in Bavaria for centuries but is now allowed.
So the law now states that malted grains, hops, water and yeast may be used - but nothing else.
What about EU regulations?
Beers brewed according to the Reinheitsgebot have special status under European Union laws as a protected traditional foodstuff.
However European law means that the German brewing industry has had to accept that beers brewed elsewhere not in accordance with the Reinheitsgebot can be sold in the country.