For example, one divided by two (one half) may be written as 0.5 or 0,5. Numbers consisting of long sequences of digits can be made more readable by separating them into groups, preferably groups of three, separated by a small space. For this reason, ISO 31-0 specifies that such groups of digits should never be separated by a comma or point, as these are reserved for use as the decimal sign. For example, one million (1000000) may be written as 1 000 000. For numbers whose magnitude is less than 1, the decimal sign should be preceded by a zero. The multiplication sign is either a cross or a half-height dot, though the latter should not be used when the dot is the decimal separator. Expressions Unit symbols follow the numerical value in the expression of a quantity. Numerical value and unit symbol are separated by a space. This rule also applies to the symbol "°C" for degrees Celsius, as in "25 °C", and to the percent sign, as in "10 %". The only exceptions are the symbols for the units of plane angle: degree, minute of arc, and second of arc – which follow the numerical value without a space in between (for example "30°"). Where quantities are added or subtracted, parenthesis can be used to distribute a unit symbol over several numerical values, as in T = 25 °C − 3 °C = (25 − 3) °C P = 100 kW ± 5 kW = (100 ± 5) kW (but not: 100 ± 5 kW) d = 12 × (1 ± 10−4) m Products can be written as ab, a b, a⋅b, or a×b. The sign for multiplying numbers is a cross (×) or a half-height dot (⋅). The cross should be used adjacent to numbers if a dot on the line is used as the decimal separator, to avoid confusion between a decimal dot and a multiplication dot. Division can be written as , a/b, or by writing the product of a and b−1, for example a⋅b−1. Numerator or denominator can themselves be products or quotients, but in this case, a solidus (/) should not be followed by a multiplication sign or division sign on the same line, unless parentheses are used to avoid ambiguity. Mathematical signs and symbols A comprehensive list of internationally standardized mathematical symbols and notations can be found in ISO 31-11. See also