What is the "solera system", used in the production of sherry?

Sherry has a special and rather complex system of maturation using a large number of casks and fractional blending. This system is called “solera” and it is used in the production of all types of sherry, dry or sweet. It is also commonly used for other wines, Spanish brandy, sherry vinegar, Madeira and Port wines and occasionally other drinks like whisky or beer. The base idea is fractional blending, where younger wines in upper rows of casks are used to top up casks of older wines stored below in order to produce a consistently aged blend.

The dynamic ageing method involving fractional blending probably originated during the second part of the 18th century in Sanlúcar de Barrameda near Cádiz in southern Spain; shortly afterwards it was developed in Jerez. Before this method was developed, all sherries were bottled as “añadas” or vintage wines, a concept that was still widely in use until the 20th century. At first the nethod consisted of blending wine from the previous harvest (then called “vino añejo”) with the new production, gradually evolving to a system of blending statically aged wines of different ages. Each wine was given a numeric value from 1 to 8, related to its average age.

By the mid 19th century the "solera" system was used as the current practice of dynamically ageing wine in gradually evolving stages with regular, systematic refreshments. This also marks the beginning of the terminology “solera y criaderas” ("ground level and tiers").