Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit (1686–1736). Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees. The Fahrenheit scale was replaced by the Celsius scale in most countries during the mid to late 20th century, though Canada retains it as a supplementary scale that can be used alongside Celsius. Fahrenheit remains the official scale of the United States, Cayman Islands and Belize. The Rankine temperature scale was based upon the Fahrenheit temperature scale, with its zero representing absolute zero instead.

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F (at standard atmospheric pressure). This puts the boiling and freezing points of water exactly 180 degrees apart. Therefore, a degree on the Fahrenheit scale is ¹⁄₁₈₀ of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart. A temperature interval of 1 °F is equal to an interval of ⁵⁄₉ degrees Celsius. The Fahrenheit and Celsius scales intersect at −40° (−40 °F and −40 °C represent the same temperature).

Li You (李祐) (died June 22, 829), courtesy name Qingzhi (慶之), was a general of the Chinese dynasty Tang Dynasty. He started his career serving under the warlord Wu Yuanji but, after his capture by the imperial general Li Su in Li Su's campaign against Wu, became instrumental in Li Su's victory over Wu. He subsequently served the imperial government until his death immediately following his campaign against the warlord Li Tongjie.

Little is known about Li You's background, including when he was born or where his family was from. It is known that he came to serve as an officer under Wu Yuanji, who controlled Zhangyi Circuit (彰義, headquartered in modern Zhumadian, Henan), and fought in Wu's campaign of resistance against the imperial government after the imperial government refused to sanction Wu Yuanji's takeover of the circuit after the death of his father Wu Shaoyang in 814. It was said that Li You was fierce in battle, and that the imperial forces feared him. He was also said to have inflicted many casualties on the imperial forces during the campaign.