It says that beer should only be made from water, malt and hops. Nothing more!
Beer
The Reinheitsgebot was the German purity law and it allowed only water, barley, and hops. It has been replaced with the Provisional German Beer Law of 1993, which allows but does not require, other items such as wheat and cane sugar.
Is this a quiz? The German Rheinheitsgebot only allows water, barley malt and hops in beer. The barley in the US has a higher protein level than European barleys, so adjuncts help. Most US brewers use corn or rice in addition to water, barley malt and hops.
50% of beer is made of water...but, that's like saying 50% of people have a nose--so do the other 50%. Beer is made from grains, yeast, flavorants and water. Some beers have rice, some have wheat, some have barley. Some beers have hops added for flavor. German beers used to have to abide by the German Beer Purity law, Reinheitsgebot, which prohibited anything besides water, barley, hops and yeast (after Louis Pasteur identified it). The best guess one might come up with to answer your question would be "Barley". Mostly, barley is found in German and German-influenced beers.
Traditional brewing of beer never used corn. Only when Americans began to brew did corny beer enter the mainstream market. In fact, by German purity laws, corn can not be an ingredient in anything called beer.
Any Beer made according to the German Beer Purity laws (Rheinheitsgebot) is not permitted to contain any ingredient other than Malt, Hops, Yeast, and Water. So any German Beer which is legal to sell in Germany will contain no starch additives. There are many other beers which also steer clear of starch stretchers - but you will always be safe with native German brews.
Reinheit
German who started brewing beer in Dunedin in 1857
Bier
A beer hall is a large German public house which specializes in beer.
Smoked beer, which is often used in a German beer festival.
The Lambert-Beer law is the base of absorption spectrophotometry.