Far fewer people know that 0° and 100° in Fahrenheit also correspond to specific real-world values. 0°F corresponds to a temperature where a brine is made of equal parts ice, water, and ammonium chloride. Such a brine, interestingly, is a frigorific mixture, meaning that it stabilizes to a specific temperature regardless of the temperature that each component started at. Thus, it makes for a really nice laboratory-stable definition of a temperature. Similarly, 100°F was initially set at “blood heat” temperature, or the human body temperature. While not super precise, it was a fairly stable value. As good as anything in the early 1700s.

Source from a quick Google search: https://gregable.com/2014/06/temperature-scales.html