Traditionally, they insisted on maturing their spirit solely in the highest quality sherry casks, sourced directly from Jerez, Spain:  in fact, the Edrington Group, which owns the brand, has even invested in sherry vineyards to ensure the continuing supply of their precious oak, thus giving them the oversight they need to maintain quality.  The exclusive use of sherry casks naturally drove up the price of the resulting whisky, particularly as sherry casks have become rarer and more expensive due to the falling popularity of sherry as a drink.  In 2005, Macallan finally released a range of whiskies matured in bourbon cask wood (the “Fine Oak” series), but it is the classic heavily-sherried style that remains their trademark.