The appellations

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According to the Bureau National Interprofessionnel du Cognac (BNIC), the official quality grades of cognac are the following:

V.S. (Very Special) or ★★★, (three stars) , De Luxe, Sélection ou Millésime
Designates a blend in which the youngest brandy has been stored for at least two years in cask.
V.S.O.P. (Very Superior Old Pale), Réserve, Vieux, Rare, Royal
Designates a blend in which the youngest brandy is stored for at least four years in a cask.
X.O. (Extra Old), V.O. (Very Old), Napoléon, Très Vieille Réserve, Héritage, Excellence, Suprême
Currently designates a blend in which the youngest brandy is stored for at least six years. 
The minimum storage age of the youngest brandy used in an X.O. blend will be increased to ten years in April 2018; this rule was originally scheduled for implementation in 2016 but was postponed due to inadequate stocks. 
The Napoleon designation, previously unofficial, will then be used to specifically denote those blends with a minimum age of six years that do not meet the revised X.O. definition.
X.O. (Extra Old), Hors d'âge (Beyond Age), Extra, Ancestral, Impérial ou Gol
Designates a blend in which the youngest brandy is stored for at least ten years in a cask.
Hors d'âge is a designation which BNIC states is equal to XO, but in practice the term is used by producers to market a high-quality product beyond the official age scale.

The names of the grades are in English because the historical cognac trade, particularly in the 18th century, significantly involved the British.

Source : wikipedia.org