The Bavarian "Reinheitsgebot" (Purity Law) was issued on 23rd April 1516 at a convention in Ingolstadt by the two common regents of Bavaria, the dukes William IV and Ludwig X, and sets that only barley, hop and water are allowed for beer's production. Yeast is not mentioned at all because in 1516 nobody owned the knowledge about its substantial part for fermentation and that therefore it is the fourth necessary raw material for beer. The Purity Law is the last chapter of a protracted development of resolutions, which lasted over 200 years, in order to regulate the production of the beer, so that neither unhealthy adjunctions nor for bread's production relevant grain were added. Furthermore it determined all prices related to beer and secured thereby a basic foodstuff for the Bavarian people. In the course of the centuries this brewing regulation has been taken over gradually by all German countries and, since 1906, by the whole First German Empire and its successors until the modern Germany. Still today the German Beer Law is based on the Bavarian Purity Law, which still is an indication of outstanding quality.
Translation from German of the "Reinheitsgebot".
How beer is to be brewed and poured out across the land:
[…]We decree, establish and ordain at the behest of the Lords of Bavaria that henceforth in all the land, in the countryside as well as our towns and marketplaces, there is no other policy than this: From Michaelmas until the Feast of St George, one mug(1) or 'head'(2) of beer will not be sold for more than one Munich penny; and from the Feast of St George until Michaelmas, a mug will not be sold for more than two pennies of the same reckoning, and a head for no more than three heller(3), under pain of penalty. But when one brews any beer (other than Marzenbier), it will under no circumstances be poured or sold for more than one penny per mug. Further we decree that henceforth in all our towns, marketplaces and the whole of the countryside, no beer shall contain or be brewed with more ingredients than grain, hops, and water. He who knowingly violates these laws will be summarily fined a keg of beer, each time it happens[…].
(1)Mug = (Bavarian) 1.069 Liters
(2)Head = round container for fluids, containing slightly less than one Bavarian 'mug'
(3)Heller = Munich half-penny