Also brings to mind balsamic vinegar: there's a battery of casks, decreasing in size, and each year a portion of the biggest cask are transferred to the one bel

Also brings to mind balsamic vinegar: there's a battery of casks, decreasing in size, and each year a portion of the biggest cask are transferred to the one below it, part of which has been moved to the next smaller-sized cask, and so on. New grape must is then added only to the biggest. The smallest cask is where the final product comes from (after a minimum of 12 years, for the "real thing"). Having some older vinegar in each of the casks significantly enriches the flavor, but also protects against bad years, since if the flavor of the grapes is a bit off one year, it gets balanced by what's already there as well as by next year's batch.