The Celsius and Fahrenheit scales are based on the scales of Anders Celsius and D.G.Fahrenheit. Both today use fixed points of the triple point and the boiling point of water. (The triple point is where water vapour, ice and liquid are in equilibrium and is within a tenth of a degree of the more difficult to measure freezing point). On the Celsius scale these fixed points are 0 (triple point) 100 (boiling point) On the Fahrenheit scale these fixed point are 32 (triple point) 212 (boiling point) Celsius = (Fahrenheit - 32)*5/9 Fahrenheit = Celsius*9/5 + 32 The two scales agree at -40. I.e. -40 F = -40 C