Flor is the first element unique to sherry; the solera aging system is the second. This special aging method was thought up to balance the characters of the different wines. In principle, long lines of casks are stacked on top of each other at least three casks high. This stack is called the solera, and each layer of barrels is called a criadera. When the time comes to bottle the wine, one third of the contents of the lowest cask in the solera is siphoned off; the cask is then topped off with the same amount of wine from the cask immediately above it in the solera. Similarly, each criadera is replenished with wine from the "younger" criadera above it. The barrels at the top of the solera are topped off with wine from the most recent vintage. This process unifies the aromas and provides a consistency which makes them unique.
The Finos are required to pass through a minimum of three criaderas before bottling, but it is possible to find complex soleras with as many as 14 levels. If the flor dies during the solera aging process the wine becomes an amontillado. It is refortified to prevent future flor development and transferred to a separate solera for further aging.