Across the globe, all whiskies share a common bond: each must be distilled from a mash (def. to heat/cook in water) of grain. The primary grains used in whiskey today are barley, corn, rye, and wheat. No other spirit has this requirement.
Vodka? Nope! Potatoes, grains, or anything to make a neutral, flavorless spirit.
Gin? Negative. Juniper berries.
Liqueur? Also no. Its an spirits orgy free for all. Pretty much do whatever but add a cool label!
Why grain for whiskey? Besides it being a law, there are two historical factors: abundance and ease of harvesting in the areas where each whiskey began.
Barley is readily harvested in the Scotland and Ireland. As a result, Scotch & Irish Whiskey have a barley foundation. Japanese whiskey also has a barley foundation, although this is because they learned the practice from the Scots.
In the U.S., rye was the original king grain of whiskey as it thrives in the Pennsylvania climate. As the population spread south to Kentucky, the new settlers discovered the potential for corn agriculture (thanks to Native Americans) and thus explains bourbon’s roots.
BOOM! Another shot of knowledge!