Constant equals. The mathematical symbol written by three horizontal lines: ≡, which is used when some parameter variables are always a constant or a constant expression, always equals that the relationship has nothing to do with the variables. "Equal to" is generally conditional and needs to meet certain conditions before it can be established; "Constant equals" is unconditional and holds true under any circumstances.

In the mathematics books of 15 and 16 centuries, words were also used to express the equal relationship between the two quantities. For example, in some formulas at that time, the word aequ or aequaliter was often written, meaning "equal". 1557, British mathematician Colder said in his paper "The Grindstone of Wisdom": In order to avoid boring repetition of the word isaequalleto (equal to), I carefully compared many figures and symbols, and felt that there were no parallel equal-length line segments in the world with the same meaning.

Therefore, Colder creatively uses two parallel and equal line segments "=" to represent "equality", and "=" is called equal sign. Replacing this word with "=" means equality, which is a progress in mathematics. Limited by the historical conditions at that time, the equal sign invented by Colder was not immediately adopted by everyone.