The quart is a unit of volume equal to a quarter of a gallon. Three kinds of quarts are currently used: the liquid quart and dry quart of the US customary system and the imperial quart of the British
One-quart milk jugs (top shelf); half-gallon (two-quart) milk jugs (bottom shelf)
The gallon is a unit of volume in British imperial units and United States customary units.
Quart
…The quart (symbol: qt) is a unit of volume equal to a quarter of a gallon. Three kinds of quarts are currently used: the liquid quart and dry quart of the US customary system and the imperial quart of the British imperial…
A 1-US-gallon gasoline can showing "U.S. Gallon" marking (for American use), imperial gallons (for some Caribbean countries' and territories' use), and litres (for Canadian use)
A Shell petrol station selling 2* and 4* (leaded petrol) by the gallon in the UK, circa 1980
A fuel station in the United States displaying fuel prices per US gallon
A fuel station in St Vincent and the Grenadines displaying prices in Eastern Caribbean dollars per imperial gallon