The equals sign or equality sign (=) is a mathematical symbol used to indicate equality. It was invented in 1557 by Robert Recorde. In an equation, the equals sign is placed between two (or more) expressions that have the same value. In Unicode and ASCII, it is U+003D = EQUALS SIGN (HTML
The etymology of the word "equal" is from the Latin word "æqualis" as meaning "uniform", "identical", or "equal", from aequus ("level", "even", or "just").
The "=" symbol that is now universally accepted in mathematics for equality was first recorded by Welsh mathematician Robert Recorde in The Whetstone of Witte (1557). The original form of the symbol was much wider than the present form. In his book Recorde explains his design of the "Gemowe lines" (meaning twin lines, from the Latin gemellus):
And to auoide the tediouſe repetition of theſe woordes : is equalle to : I will ſette as I doe often in woorke vſe, a paire of paralleles, or Gemowe lines of one lengthe, thus: =, bicauſe noe .2. thynges, can be moare equalle.
And to avoid the tedious repetition of these words: is equal to: I will set as I do often in work use, a pair of parallels, or Gemowe lines of one length, thus: =, because no 2 things, can be more equal.
The symbol '=' was not immediately popular. The symbol || was used by some and æ (or œ), from the Latin word aequalis meaning equal, was widely used into the 1700s.
In mathematics, the equals sign can be used as a simple statement of fact in a specific case (x = 2), or to create definitions (let x = 2), conditional statements (if x = 2, then …), or to express a universal equivalence
(x + 1)2 = x2 + 2x + 1.
The first important computer programming language to use the equals sign was the original version of Fortran, FORTRAN I, designed in 1954 and implemented in 1957. In Fortran, "=" serves as an assignment operator:
X = 2 sets the value of
X to 2. This somewhat resembles the use of "=" in a mathematical definition, but with different semantics: the expression following "=" is evaluated first and may refer to a previous value of
X. For example, the assignment
X = X + 2 increases the value of
X by 2.
A rival programming-language usage was pioneered by the original version of ALGOL, which was designed in 1958 and implemented in 1960. ALGOL included a relational operator that tested for equality, allowing constructions like
if x = 2 with essentially the same meaning of "=" as the conditional usage in mathematics. The equals sign was reserved for this usage.
Both usages have remained common in different programming languages into the early 21st century. As well as Fortran, "=" is used for assignment in such languages as C, Perl, Python, awk, and their descendants. But "=" is used for equality and not assignment in the Pascal family, Ada, Eiffel, APL, and other languages.
A few languages, such as BASIC and PL/I, have used the equals sign to mean both assignment and equality, distinguished by context. However, in most languages where "=" has one of these meanings, a different character or, more often, a sequence of characters is used for the other meaning. Following ALGOL, most languages that use "=" for equality use ":=" for assignment, although APL, with its special character set, uses a left-pointing arrow.
Fortran did not have an equality operator (it was only possible to compare an expression to zero, using the arithmetic IF statement) until FORTRAN IV was released in 1962, since when it has used the four characters ".EQ." to test for equality. The language B introduced the use of "==" with this meaning, which has been copied by its descendant C and most later languages where "=" means assignment.
In PHP, the triple equals sign (
===) denotes value and type equality, meaning that not only do the two expressions evaluate to equal values, but they are also of the same data type. For instance, the expression
0 == false is true, but
0 === false is not, because the number 0 is an integer value whereas false is a Boolean value.
== cannot be described by any simple consistent rules. The expression
0 == false is true, but
0 == undefined is false, even though both sides of the
== act the same in Boolean context. For this reason it is sometimes recommended to avoid the
In Ruby, equality under
== requires both operands to be of identical type, e.g.
0 == false is false. The
=== operator is flexible and may be defined arbitrarily for any given type. For example, a value of type
Range is a range of integers, such as
(1800..1899) == 1844 is false, since the types are different (Range vs. Integer); however
(1800..1899) === 1844 is true, since
Range values means "inclusion in the range". Under these semantics,
=== is non-symmetric; e.g.
1844 === (1800..1899) is false, since it is interpreted to mean
Integer#=== rather than
The equals sign is also used as a grammatical tone letter in the orthographies of Budu in the Congo-Kinshasa, in Krumen, Mwan and Dan in the Ivory Coast. The Unicode character used for the tone letter (U+A78A) is different from the mathematical symbol (U+003D).
A possibly unique case of the equals sign of European usage in a person's name, specifically in a double-barreled name, was by pioneer aviator Alberto Santos-Dumont, as he is also known not only to have often used a double hyphen resembling an equals sign (=) between his two surnames in place of a hyphen, but also seems to have personally preferred that practice, to display equal respect for his father's French ethnicity and the Brazilian ethnicity of his mother.
Instead of a double hyphen, the equals sign is sometimes used in Japanese as a separator between names. In Ojibwe, the readily-available equal sign on a keyboard is used as a substitute for a double hyphen.
In recent years, the equals sign has been used to symbolize LGBT rights. The symbol has been used since 1995 by the Human Rights Campaign, which lobbies for marriage equality, and subsequently by the United Nations Free & Equal, which promotes LGBT rights at the United Nations.
The triple bar symbol "≡" (U+2261, LaTeX \equiv) is often used to indicate an identity, a definition (which can also be represented by U+225D "≝" or U+2254 "≔"), or a congruence relation in modular arithmetic.
Additional symbols in Unicode related to the equals sign include:
The equals sign is sometimes used incorrectly within a mathematical argument to connect math steps in a non-standard way, rather than to show equality (especially by early mathematics students).
For example, if one were finding the sum, step by step, of the numbers 1, 2, 3, 4, and 5, one might incorrectly write
Structurally, this is shorthand for
but the notation is incorrect, because each part of the equality has a different value. If interpreted strictly as it says, it implies
A correct version of the argument would be
This difficulty results from subtly different uses of the sign in education. In early, arithmetic-focused grades, the equals sign may be operational; like the equals button on an electronic calculator, it demands the result of a calculation. Starting in algebra courses, the sign takes on a relational meaning of equality between two calculations. Confusion between the two uses of the sign sometimes persists at the university level.