Fahrenheit proposed his temperature scale in 1724, basing it on two reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point was determined by placing the thermometer in "a mixture of ice, water, and salis Armoniaci [transl. ammonium chloride] or even sea salt". This combination forms a eutectic system which stabilizes its temperature automatically: 0 °F was defined to be that stable temperature. A second point, 96 degrees, was approximately the human body's temperature (sanguine hominis sani, the blood of a healthy man). A third point, 32 degrees, was marked as being the temperature of ice and water "without the aforementioned salts".

According to a German story, Fahrenheit actually chose the lowest air temperature measured in his hometown Danzig (Gdańsk, Poland) in winter 1708/09 as 0 °F, and only later had the need to be able to make this value reproducible using brine.