According to the specifications of the Cognac Appellation d'Origine Contrôlée, the eaux-de-vie must be aged without interruption exclusively in oak barrels, for a minimum of 2 years (VS) before they can be sold.
Traditionally, two types of oak are used to make Cognac ageing barrels: Sessile Oak, which is generally fine-grained, and Pedunculate Oak, which is generally coarse-grained. These two species are very common in Europe.
For some years now, after this passage in oak barrels, of varying lengths, some Cognac houses have used barrels that have been used for other alcohols (Monbazillac, Sauternes, etc.), or oak barrels of a different species (American, Japanese) to continue the ageing of the eaux-de-vie for a few more months and to give it a different "finish".