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Main articles: "SI, "SI base unit, "SI derived unit, and "SI prefix
• If an internationally standardized symbol exists for a unit, then only that symbol should be used. See the "SI articles for the list of standard symbols defined by the International System of Units. Note that the distinction between uppercase and lowercase letters is significant for SI unit symbols. For example, "k" is the prefix kilo and "K" stands for the unit kelvin. The symbols of all SI units named after a person or a place start with an uppercase letter, as do the symbols of all prefixes from mega on upwards. All other symbols are lowercase; the only exception is "litre, where both l and L are allowed. However, it is stated that the CIPM will examine whether one of the two may be suppressed.
• Symbols for units should be printed in an upright ("roman) typeface.

### Numbers

See Sect. 3.3 of the Standard text.

• Numbers should be printed in upright ("roman) type.
• ISO 31-0 (after Amendment 2) specifies that "the "decimal sign is either the comma on the line or the point on the line". This follows resolution 10[1] of the 22nd "CGPM, 2003.[2]
• 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 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". The only exception are the symbols for the units of plane angle degree, minute and second, 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, ab, 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 ${\displaystyle {\frac {a}{b}}}$, a/b, or by writing the product of a and b−1, for example ab−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.