The contents of this batch were distilled using copper double-column stills, primarily from 1984 plantings that were harvested in late 1993 and placed into barrels in early 1994. These casks were stored in a dry, earthen-floor room in Atotonilco el Alto for a decade before being relocated to a higher, cooler climate above the highland town, where they rested for an additional 11 years. The vast majority of the barrels used were previously employed for aging California white wine, while one barrel had held Canadian rye whiskey. Master distiller Enrique Fonseca's objective was to create the world's longest-aged tequila, and he accomplished this feat with this particular blend. Unlike most other releases in the 2015 series, which feature at least some pot-stilled tequila, the 21-year-old expression is crafted exclusively with copper double-column stills. The majority of the spirit was aged in barrels that had previously held Canadian rye whiskey, and one cask had previously held California white wine. After spending ten years in Atotonilco el Alto, the barrels were transported to Chapingo, where they aged for an additional eleven years.