Criaderas and solera is a unique storage system. This is the traditional aging system that is used for sherry. The butts—the casks in which the wine is stored—are stacked up high in rows, one on top of another, and connected to each other. Those on the top row (the first criadera) contain the youngest wine; those in the middle row (the second criadera), the oldest wine; and those on the bottom (the solera), the vintage wine, which is a mixture of other wines. Wine ready for consumption is taken from the solera (bottom row). The casks are refilled with wine from the top row and the casks on the top row with new wine. This makes the solera a complex mixture as a result of the number of vintages that it contains, since it is not possible to know which year the wine dates from, although it does have an average age of 20 or 30 years.