In 1855, Napoleon III asked the Bordeaux Chamber of Commerce to prepare an exhibition of the region's finest wines for the Paris Universal Exhibition. The merchants (courtiers) — who had been informally ranking Bordeaux châteaux by price in their daily trading for over a century — were asked to formalise this into an official hierarchy. They did so in approximately one week, based almost entirely on the prices their wines had commanded in the marketplace over the preceding decades.
The result was a list of 61 red wine châteaux from the Médoc peninsula (plus one from the Graves region, Haut-Brion), ranked in five tiers from Premier Cru Classé through Cinquième Cru Classé, and a separate classification of Sauternes and Barsac sweet wines. The document was a snapshot of commercial standing in 1855, not a scientific or agronomic assessment.
Bordeaux is a region of approximately 120,000 hectares divided by the Gironde estuary and Dordogne river into two fundamentally different zones: the Left Bank (Médoc and Graves — Cabernet Sauvignon dominant, on well-drained gravel ridges called graves) and the Right Bank (Pomerol and Saint-Émilion — Merlot dominant, on clay and limestone). The 1855 classification covers only the Left Bank. The Right Bank's wines — including Pomerol's Pétrus and Saint-Émilion's Angélus and Pavie — operate under entirely different classification systems, or none at all.