Editing Equality (mathematics) (section)

== Etymology ==
{{Further|Equals sign#History}}
[[File:First Equation Ever.png|thumb|The first use of an [[equals sign]], equivalent to <math>14x+15=71</math> in modern notation. From ''[[The Whetstone of Witte]]'' (1557) by [[Robert Recorde]].|upright=1.2]]
[[File:Recorde - The Whetstone of Witte - equals.jpg|thumb|Recorde's introduction of {{Char|1==}}. "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 twin lines of one [the same] length, thus: ==, because no 2 things can be more equal."<ref name="Recorde1557">{{cite book |last=Recorde |first=Robert |title=The Whetstone of Witte |year=1557 |url=https://archive.org/details/TheWhetstoneOfWitte/page/n237/mode/2up?view=theater |at=p. 3 of "The rule of equation, commonly called Algebers Rule" |author-link=Robert Recorde |location=London |publisher=Jhon Kyngstone |ol=17888956W}}</ref>|upright=1.2]]
In English, the word ''equal'' is derived from the [[Latin]] {{lang|la|aequālis}} ('like', 'comparable', 'similar'), which itself stems from {{lang|la|aequus}} ('level', 'just').<ref>{{Cite dictionary |title=Equal |url=https://www.merriam-webster.com/dictionary/equal |access-date=2020-08-09 |dictionary=[[Merriam-Webster]] |archive-date=2020-09-15 |archive-url=https://web.archive.org/web/20200915001915/https://www.merriam-webster.com/dictionary/equal |url-status=live}}</ref> The word entered [[Middle English]] around the 14th century, borrowed from [[Old French]] {{lang|fro|equalité}} (modern {{lang|fr|égalité}}).<ref>{{Cite dictionary |title=Equality |url=https://www.etymonline.com/word/equality |access-date=2024-12-16 |dictionary=Etymonline}}</ref> More generally, the interlingual synonyms of ''equal'' have been used more broadly throughout history (see {{Section link||Geometry}}).

Before the 16th century, there was no common symbol for equality, and equality was usually expressed with a word, such as ''aequales, aequantur, esgale, faciunt, ghelijck,'' or ''gleich,'' and sometimes by the abbreviated form ''aeq'', or simply {{angbr|æ}} and {{angbr|œ}}.{{Sfn|Cajori|1928|pp=298–305}} [[Diophantus]]'s use of {{angbr|ἴσ}}, short for {{lang|grc|ἴσος}} ({{tlit|grc|ísos}} 'equals'), in ''[[Arithmetica]]'' ({{circa|250&nbsp;AD}}) is considered one of the first uses of an [[equals sign]].<ref>{{cite book |last=Derbyshire |first=John |author-link=John Derbyshire |url=https://archive.org/details/isbn_9780309096577/page/34/mode/2up?q=Equals |title=Unknown Quantity: A Real And Imaginary History of Algebra |publisher=Joseph Henry Press |year=2006 |isbn=0-309-09657-X |page=35}}</ref>

The sign {{Char|1==}}, 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 symbol as "Gemowe lines", from the Latin {{lang|la|gemellus}} ('twin'), using two [[parallel lines]] to represent equality because he believed that "no two things could be more equal."<ref name="Recorde1557" />{{Sfn|Cajori|1928|pp=298–305}}

Recorde's symbol was not immediately popular. After its introduction, it wasn't used again in print until 1618 (61 years later), in an anonymous Appendix in [[Edward Wright (mathematician)|Edward Wright's]] English translation of ''[[Mirifici Logarithmorum Canonis Descriptio|Descriptio]]'', by [[John Napier]]. It wasn't until 1631 that it received more than general recognition in England, being adopted as the symbol for equality in a few influential works. Later used by several influential mathematicians, most notably, both [[Isaac Newton]] and [[Gottfried Leibniz]], and due to the prevalence of [[History of calculus#Newton and Leibniz|calculus at the time]], it quickly spread throughout the rest of Europe.{{Sfn|Cajori|1928|pp=298–305}}