Magnums

A magnum holds two bottles' worth but the reason to buy one is not arithmetic.
The pocket of air trapped under the cork is the same size as in a standard bottle which means there is half as much oxygen for every glass of wine sitting beneath it. Wine in magnum ages slower and more evenly as a result. That's why growers tend to reserve their better cuvées for this format and why an old magnum so often shows better than an old bottle of the same wine. Besides aging, they are also the most straightforward way to feed a table without anybody having to get up and open something else.