Rye
To be able to be called Rye Whiskey, a whiskey must be made from at least 51% rye.

Early settlers to the New World brought their whiskey making expertise and desire with them. While the specific grains and ingredients that they used in Ireland and Scottland for this process were not readily found or cultivated on this new soil, they were able to grow Rye, which they soon began turning into American rye whiskey by the mid-1700's.

Today, Rye whiskey is not as common as it once was. And prohibition is to blame for this. During these dry years, virtually all (legal) American distilleries were shut down. Once prohibition was repealed, the distilleries fired back up again, but whiskey requires aging in order to mellow its bite, and there was no properly aged product available, and wouldn't be for several years.