Which two types of bitters are commonly used in cocktails?

The two types of bitters commonly used in cocktails, Angostura and Peychaud's, are renowned for their distinct flavor profiles and historical significance in mixology. Angostura bitters, originally developed in the early 19th century, offers a complex flavor that includes aromatic spices and herbs, making it a versatile ingredient in many classic cocktails such as the Old Fashioned and the Manhattan. Its concentrated flavor adds depth and balance to drinks.

Peychaud's bitters, on the other hand, hails from New Orleans and is characterized by a lighter, more floral taste with an anise undertone, making it particularly suited for cocktails like the Sazerac. The combination of these two bitters in drinks enhances the overall flavor complexity, providing bartenders with tools to craft well-rounded cocktails.

The other options present alternatives that are either less traditional or specific to certain cocktails but do not have the same widespread use and recognition in cocktail culture as Angostura and Peychaud's. For instance, while orange bitters are gaining popularity, particularly in modern mixes, they do not hold the same foundational status in cocktail creation as the bitters mentioned in the correct answer.