The region of Cognac lies in the two departments of The Charente and Charente-Maritime, just north of the Médoc. Its 2,798,447 acres were first defined by law in 1909, followed by the delimitation of its seven "crus" in 1936. Of these, the Grande Champagne was ranked first, confirming the superiority it had historically enjoyed. Though it represents only 3.2 percent of the total Cognac appellation, it is the most extensively planted, with 32,904 acres of vines representing 15.6 percent of Cognac's total vineyard acreage, testimony both to its quality and rarity. While the soils of Cognac are generally characterized by chalk, the degree and quality of the chalkiness varies in each cru. Of the three types of chalk which surface in the area, that referred to as "Campanian" (the Latin derivation of "Champagne") occurs only on the crests of the gentle slopes of the Grande Champagne. It is composed almost entirely of a variety of oyster fossil found nowhere else, the "ostrea vesicularis," which has degraded to a crumbly, grey-white stratum with traces of silica and crystallized iron pyrite. It is this soil which makes Cognac unique among the world's brandies. Nine white vine varieties, also defined in 1936, are authorized in the production of wines destined to become Cognac. The Ugni Blanc, once known as the Saint-Emilion des Charentes, accounts for approximately 95 percent of plantings, and yields a thin, acidic white wine. The only requirement of this base product is that it be "correct;" indeed, the more characterless the wine, the finer the Cognac. The wine must be double-distilled by the 31st of March following the harvest in the traditional "alembic charentais," a copper pot still heated by an open flame, the design of which has changed little since the early 1700s. The clear brandy resulting from the heart of the second distillation, or "bonne chauffe," has been reduced to one-tenth its original volume with a alcoholic concentration of 72 percent.