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.
Vodka is generally made from grains, rye, potatoes, wheat, sugar beet, grapes and molasses. One of the main differences that can be seen between vodka and beer is that the former one is a distilled beverage and the latter one is a fermented one. When comparing the calorie content, vodka has fewer calories than beer.