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 traditionally is made from grains (usually the gluten grains wheat, barley, and rye). But there's a growing slate of specialty vodkas made from alternative materials such as corn, potatoes, and grapes ... and there's some evidence that these vodkas may fit into a gluten-free diet better than traditional vodka options.