Which ingredient is NOT included in a Regular Rob Roy?

A Regular Rob Roy is a classic cocktail that traditionally consists of Scotch whiskey, sweet vermouth, and bitters. The inclusion of Scotch whiskey is what defines the drink, making it a variation of a Manhattan that uses Scotch instead of bourbon or rye. Sweet vermouth adds sweetness and complexity, while bitters provide depth and balance to the flavors.

Bourbon, on the other hand, is not included in a Regular Rob Roy. This whiskey type is distinct from Scotch, which is made from malted barley and produced in Scotland. As such, using bourbon would result in a different cocktail entirely, shifting it away from its traditional Scotch roots. Therefore, the main ingredient that distinguishes a Regular Rob Roy from other similar cocktails, such as a Manhattan, is the use of Scotch, not bourbon.