Alton uses a quarter cup of Red Wine Vinegar in his vinaigrette.
The cup is a convenient unit of measurement for volume, used in cooking to measure liquids (fluid measurement) and bulk foods such as granulated sugar (dry measurement) in many countries, although different countries use different sizes. This measure is usually used as an informal unit in cooking recipes rather than as a measure for the sale of foodstuffs; precision is rarely required. The United States defines a legal cup used for nutritional labelling of foodstuffs.
Actual cups used in a household in any country may differ from the cup size used for recipes; standard measuring cups, often calibrated in fluid measure and weights of usual dry ingredients as well as in cups, are available.