The symbol means that the number (to the right of the symbol) is really two numbers: a positive number AND a negative number.

For Example, 8 stands for two numbers:

+8 and -8

The symbol is commonly used:

(1) In mathematical expressions and equations.

(2) To quantify the margin or error (precision) of a computation or measurement.

Example 1: , use in Mathematical Expressions and Equations

Square Root:

The use of the plus-minus symbol is a short hand way of showing that the square root of 256 has two answers:

Absolute Value:

This absolute value equation can be rewritten using the plus-minus symbol:

The use of the plus-minus symbol clearly shows that two equations must be solved to determine the values of x which satisfy the original equation. These equations are:

Quadratic Equation:

The use of the plus-minus symbol shows that two equations must be solved to determine which values of x satisfy the quadratic equation. These equations are:

(1) Using the "plus" sign:

(2) Using the "minus" sign:

Example 2: , use in "Precision" or "Margin or Error"

Margin of Error:

A local poll taken May 1-4 of 1200 registered voters showed Candidate A would beat Candidate B if the election were held now, 60 to 30 percent, with seven percent undecided. This poll has a margin of error of plus or minus four percentage points.

Candidate A should receive between 56% and 64% of the vote if the election were held today.

This could be written:

Candidate A
Likely Vote

Engineering Tolerance:

A round pin is being manufactured to fit into a round hole, which is 20 mm in diameter. The round pin might have a manufacturing specification of 19.95 mm to 19.99 mm in diameter.

Using the plus-minus sign, this could be written:

Diameter of
Round Pin