The system is simplicity itself: three rows of casks are piled up on top of each other in each criadera. The name solera is derived from suelo for the wine in the bottom row, which is the oldest of the three. The top row houses the youngest Spanish wine.
As wine is drawn of for bottling this is done from the lowest casks and the space created is then filled by wine from a cask that is one row higher. The space created in this cask is then filled by wine from the next row above.