In mathematics, the sign is pronounced "plus or minus" and indicates that there are exactly two possible answers, one of which is positive and one of which is negative.
In most experimental sciences however, the sign is pronounced "give or take" and it indicates an inclusive range of values that a reading might have.
The plus-minus sign does not mean "approximately".
A percentage may also be used to indicate the error margin. For example, 230 V ± 10% refers to a voltage within 10% of either side of 230 V (207 V to 253 V).
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 could write .
The best-known example is offered by the formula for the solutions of quadratic equations:
Written out in full, this states that there are two solutions to the equation, namely
Another example is found in the trigonometric identity
This stands for two identities: one with + on both sides of the equation, and one with − on both sides.
A somewhat different use is found in this presentation of the formula for the Taylor series of the sine function:
This mild abuse of notation is meant to indicate that the sign of the terms alternate, where (starting the count at 0) the terms with an even index are added while those with an odd index are subtracted. A less ambiguous presentation in this case would use the quantity , which gives when is even and −1 when is odd.
There is another character, the minus-or-plus sign (∓), which is seen less often. It only takes on significant meaning when used in conjunction with the "±" sign. It can be used alongside "±" in such expressions as "x ± y ∓ z", which can be interpreted as "x + y − z" or/and "x − y + z", but neither "x + y + z" nor "x − y − z". The upper "−" in "∓" is considered attached to the "+" of "±" (and the lower symbols work in the same way) even though there is no visual indication of the dependency. 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.
±. The rarer minus-plus sign (∓) is not generally found in legacy encodings and does not have a named HTML entity but is available in Unicode with codepoint U+2213 and so can be used in HTML using