In linking a subordinate clause and a main clause, a relative pronoun functions similarly to a subordinating conjunction. Unlike a conjunction, however, a relative pronoun does not simply mark the subordinate (relative) clause, but also plays the role of a noun within that clause.
The main ingredient in most vodkas is some kind of grain, such as wheat, corn or rye. Potatoes are another common base ingredient for vodka, according to About.com.
Since the 1890s, the standard Polish, Russian, Ukrainian, Estonian, Latvian, Lithuanian and Czech vodkas are 40% alcohol by volume ABV (80 proof), a percentage that is widely misattributed to Dmitri Mendeleev.