Gas mark is the traditional British scale for setting the temperature of a gas oven, marked on the cooker’s control dial as a series of numbered gradations — also called Regulo numbers after an early thermostat brand. Gas Mark 4, for example, corresponds to about 180 °C (350 °F), the moderate oven of countless recipes; the scale runs from Gas Mark ¼ (very cool) up to Gas Mark 9 (very hot). British cookbooks long gave temperatures in gas marks, often alongside Celsius and Fahrenheit, and older cooks still think in them. Conversion charts are essential for anyone working across systems, since misreading the scale — mistaking Gas Mark 6 for 6 on another scale — ruins baking.