"Purity laws"

In fact the previously listed ingredients, water, barley malt, hops and yeast[1], were the only allowed beer ingredients according to the historical German Reinheitsgebot (literally 'purity requirement', or 'purity law' - if you will). Lars Marius Garshol and Ron Pattison have interesting views on it - both recommended reading.

The Reinheitsgebot originates from Bavaria, Germany (1516), but it has been enforced in several other countries up until modern times. In Norway it was enforced between 1857 and 1994. In practice this meant that brewers were constrainted with regard to what kind of beers they could make. Historically the law has been used as a means of market protection, stamp of quality assurance, and for marketing purposes. But creativity and diversity have suffered...

[1] Yeast was actually not included as constituent of beer in the original version of the law as people did not know that it existed. It was added later when it was discovered by science.