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The equal sign, equals sign, or "=" is a mathematical symbol used to indicate equality. It was invented in 1557 by Welshman Robert Recorde. The equals sign is placed between the things stated to be equal. It is Unicode character 003D

A well-known equality, featuring the equals sign.

The "=" symbol that is now universally accepted in mathematics for equality was first used by Welsh mathematician Robert Recorde in The Whetstone of Witte (1557). The original form of the symbol was much wider (longer) than the present form. In his book[1], Recorde explains his design: to auoide the tediouse repetition of these woordes: is equalle to : I will sette as I doe often in woorke use, a paire of paralleles, or Gemowe [i.e. "twin"] lines of one lengthe, thus: =, bicause noe .2. thynges, can be moare equalle. However, a manuscript from the University of Bologna, dated from 1550–1568, features the same equality symbol, possibly earlier than Recorde's use.^{[citation needed]} According to Scotland's St Andrews University Maths History website[2], "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."

A symbol used to denote items that are approximately equal is "≈" (wave lines)(Unicode character 2248), and the symbol used to denote when items are not equal is "≠" (slashed equal sign)(Unicode character 2260).

The symbol "≡" (Unicode character 2261) is often used to indicate an identity, or a congruence relation in modular arithmetic. The symbol "≘" can be used to express that an item corresponds to another.

Equality of truth values, i.e. bi-implication or logical equivalence, may be denoted by various symbols including =, ~, and <=>.

Most programming languages, which are limited to the ASCII character set, use "~=", "!=", "=/=" or "<>" to represent "not equal to"; "!=" has carried over into newsgroups and Internet forums.

In programming languages, the equals sign may either denote a boolean operator to test equality of values (sometimes a double equal sign "=="), or it may denote an assignment (sometimes denoted with a colon-equals ":="). In some programming languages such as PHP a double equals sign ("==") denotes equivalence, meaning that the variables may not be of the same data type, but their values can be reduced to the same value. The triple equal sign ("===") denotes identity[3], meaning that not only do the two values parse to be the same, they are of the same data type. For instance, in PHP the expression ("0 == false") is true, but ("0 === false") is not, because the number 0 is an integer value, whereas false is a Boolean.

• Equality (mathematics)
• Logical equality
• Plus and minus signs
• Double hyphen
• 2+2=5

• Cajori, Florian (1993). A History of Mathematical Notations. New York: Dover (reprint). ISBN 0-486-67766-4. 
• Boyer, C. B.: A History of Mathematics, 2nd ed. rev. by Uta C. Merzbach. New York: Wiley, 1989 ISBN 0-471-09763-2 (1991 pbk ed. ISBN 0-471-54397-7)

• Earliest Uses of Symbols of Relation
• Image of the page of The Whetstone of Witte on which the equal sign is introduced
• Scientific Symbols, Icons, Mathematical Symbols