# Plus–minus sign

± | |
---|---|

Plus–minus sign | |

In Unicode | U+00B1 ± PLUS-MINUS SIGN (HTML `±` · `±, ±, ±` ) |

Related | |

See also | U+2213 ∓ MINUS-OR-PLUS SIGN (HTML `∓` · `∓, ∓, ∓` ) |

The **plus–minus sign** (also, **plus or minus sign**), ± is a mathematical symbol with multiple meanings.

- In mathematics, it generally indicates a choice of exactly two possible values, one of which is the negation of the other.
- In experimental sciences, the sign commonly indicates the confidence interval or error in a measurement, often the standard deviation or standard error.
^{[1]}The sign may also represent an inclusive range of values that a reading might have. - In engineering the sign indicates the tolerance, which is the range of values that are considered to be acceptable, safe, or which comply with some standard, or with a contract.
^{[2]} - In botany it is used in morphological descriptions to notate "more or less".
- In chemistry the sign is used to indicate a racemic mixture.
- In chess, the sign indicates a clear advantage for the white player; the complementary minus or plus sign, ∓, indicates the same advantage for the black player.
^{[3]}

## History[edit]

A version of the sign, including also the French word *ou* ("or") was used in its mathematical meaning by Albert Girard in 1626, and the sign in its modern form was used as early as William Oughtred's *Clavis Mathematicae* (1631).^{[4]}

## Usage[edit]

### In mathematics[edit]

In mathematical formulas, the ± symbol may be used to indicate a symbol that may be replaced by either the Plus and minus signs, + or −, allowing the formula to represent two values or two equations.

For example, given the equation x^{2} = 9, one may give the solution as x = ±3. This indicates that the equation has two solutions, each of which may be obtained by replacing this equation by one of the two equations x = +3 or x = −3. Only one of these two replaced equations is true for any valid solution. A common use of this notation is found in the quadratic formula

describing the two solutions to the quadratic equation *ax*^{2} + *bx* + *c* = 0.

Similarly, the trigonometric identity

can be interpreted as a shorthand for two equations: one with + on both sides of the equation, and one with − on both sides. The two copies of the ± sign in this identity must both be replaced in the same way: it is not valid to replace one of them with + and the other of them with −. In contrast to the quadratic formula example, both of the equations described by this identity are simultaneously valid.

The **minus–plus sign** (also **minus-or-plus sign**), ∓ is generally used in conjunction with the ± sign, in such expressions as x ± y ∓ z, which can be interpreted as meaning x + y − z and/or x − y + z, but *not*
x + y + z *nor* x − y − z. The upper − in ∓ is considered to be associated to the + of ± (and similarly for the two lower symbols) even though there is no visual indication of the dependency. (However, the ± sign is generally preferred over the ∓ sign, so if they both appear in an equation it is safe to assume that they are linked. On the other hand, if there are two instances of the ± sign in an expression, without a ∓, it is impossible to tell from notation alone whether the intended interpretation is as two or four distinct expressions.) The original expression can be rewritten as x ± (y − z) to avoid confusion, but cases such as the trigonometric identity are most neatly written using the "∓" sign:

which represents the two equations:

Another example is

A third related usage is found in this presentation of the formula for the Taylor series of the sine function:

Here, the plus-or-minus sign indicates that the term may be added or subtracted, in this case depending on whether *n* is odd or even, the rule can be deduced from the first few terms. A more rigorous presentation of the same formula would multiply each term by a factor of (−1)^{n}, which gives +1 when *n* is even and −1 when *n* is odd.

### In statistics[edit]

The use of ± for an approximation is most commonly encountered in presenting the numerical value of a quantity together with its tolerance or its statistical margin of error.^{[1]}
For example, "5.7±0.2" may be anywhere in the range from 5.5 to 5.9 inclusive. In scientific usage it sometimes refers to a probability of being within the stated interval, usually corresponding to either 1 or 2 standard deviations (a probability of 68.3% or 95.4% in a normal distribution).

Operations involving uncertain values should always try to preserve the uncertainty in order to avoid propagation of error. If n = a ± b, any operation of the form m = f(n) must return a value of the form m = c ± d, where *c* is *f(n)* and *d* is range updated using interval arithmetic.

A percentage may also be used to indicate the error margin. For example, 230 ± 10% V refers to a voltage within 10% of either side of 230 V (from 207 V to 253 V inclusive).^{[citation needed]} Separate values for the upper and lower bounds may also be used. For example, to indicate that a value is most likely 5.7 but may be as high as 5.9 or as low as 5.6, one may write 5.7+0.2

−0.1.

### In chess[edit]

The symbols ± and ∓ are used in chess notation to denote an advantage for white and black respectively. However, the more common chess notation would be only + and –.^{[3]} If a difference is made, the symbols + and − denote a larger advantage than ± and ∓.

## Encodings[edit]

- In Unicode: U+00B1 ± PLUS-MINUS SIGN
- In ISO 8859-1, -7, -8, -9, -13, -15, and -16, the plus–minus symbol is code 0xB1
_{hex}. This location was copied to Unicode. - The symbol also has a HTML entity representations of
`±`

and`±`

. - The rarer minus–plus sign is not generally found in legacy encodings, but is available in Unicode as U+2213 ∓ MINUS-OR-PLUS SIGN so can be used in HTML using
`∓`

or`∓`

. - In TeX 'plus-or-minus' and 'minus-or-plus' symbols are denoted
`\pm`

and`\mp`

, respectively. - Although these characters may also be produced using underlining or overlining + symbol (
__+__or + ), this is deprecated because the formatting may stripped at a later date, changing the meaning. It also makes the meaning less accessible to blind users with screen readers.

### Typing[edit]

- Windows:
`Alt`+`2``4``1`or`Alt`+`0``1``7``7`(numbers typed on the numeric keypad). - Macintosh:
`⌥ Option`+`⇧ Shift`+`=`(equal sign on the non-numeric keypad). - Unix-like systems:
`Compose`,`+`,`-`or`⇧ Shift`+`Ctrl`+`u``B``1``space`(second works on Chromebook) - AutoCAD shortcut string:
`%%d`

## Similar characters[edit]

Look up , 士, or 土 in Wiktionary, the free dictionary.干 |

The plus–minus sign resembles the Chinese characters 土 (Radical 32) and 士 (Radical 33), whereas the minus–plus sign resembles 干 (Radical 51).

## See also[edit]

- Plus and minus signs
- Table of mathematical symbols
- ≈ (approximately equal to)
- Engineering tolerance

## References[edit]

- ^
^{a}^{b}Brown, George W. (1982), "Standard Deviation, Standard Error: Which 'Standard' Should We Use?",*American Journal of Diseases of Children*,**136**(10): 937–941, doi:10.1001/archpedi.1982.03970460067015, PMID 7124681. **^**Engineering tolerance- ^
^{a}^{b}Eade, James (2005),*Chess For Dummies*(2nd ed.), John Wiley & Sons, p. 272, ISBN 9780471774334. **^**Cajori, Florian (1928),*A History of Mathematical Notations, Volumes 1-2*, Dover, p. 245, ISBN 9780486677668.