Triple sec, actually a French liqueur invented in the 19th century, has become a somewhat generic term for any orange-flavored liqueur. Under this definition, both the Grand Marnier and Cointreau brands of orange liqueur are types of triple sec, although the taste and composition distinguish them from each other and traditional triple sec. All three share origins in 19th century France.
The 19th century Frenchman Jean-Baptiste Combier invented triple sec when he steeped the sun-dried skins of Haitian oranges in neutral spirits then distilled it in copper pots. Triple sec, meaning "triple distilled" in English, typically contains about 23 percent alcohol and has a mildly sweet orange flavor. The liqueurs range from clear to golden; higher quality triple secs typically use aged cognac or brandy as the base, while lower quality triple sec may use low-proof grain alcohol.
Grand Marnier, a cognac-based liqueur flavored with the aromatic Citrus bigaradia tropical orange, was introduced in 1827. The recipe remains safely guarded in the hands of the Marnier Lapostolle family in 2011. Grand Marnier ages in oak casks for up to a decade before bottling. The 40 percent alcohol by volume is high for a liqueur; fans typically enjoy Grand Marnier warmed in a snifter.
Cointreau, a brand of orange liqueur produced in the French suburb of Saint-Barthelemy d'Anjou, was first manufactured in 1875. A popular aperitif and cocktail ingredient, Cointreau is often consumed as an after-dinner digestif as well. Cointreau's flavor comes from a blend of sweet and bitter orange peels steeped in pure alcohol derived from sugar beets. Cointreau often replaces triple sec in premium margaritas. Like Grand Marnier, Cointreau also contains about 40 percent alcohol.
Triple sec typically gets blended with other liquors in mixed drinks. Cointreau and Grand Marnier are technically types of triple sec, but both contain more alcohol than traditional triple sec. Grand Marnier and Cointreau both taste less sweet than most triple secs, and although they do stand in for triple sec in some top-shelf cocktails, they're also frequently consumed on their own.