**GN01:** We *must not* put numbers in parentheses
after they have already been mentioned (e.g., five (5)).

**GN02:** We *must* use words to express numbers below 10,
*unless* the numbers precede a unit of measurement (e.g., 4.52 cm),
they are part of statistical or mathematical operations
(e.g., 5% of the participants; divided by 8),
or they denote dates, exact amounts of money,
specific places in numbered series, parts of books and tables,
or each number in a list of four or more digits.

**GN03:** We *must* express in words
numbers at the beginning of a sentence or a title
(e.g., Fourteen participants were in the control group)
and common fractions and phrases
(e.g., Three-fourths of the population).

**GN04:** We *must* separate the integer parts of decimal numbers
from their fractional parts
using decimal points, not commas.

**GN05:** We *must* place a zero before a decimal fraction less than 1
if the statistic can exceed 1 (e.g., 0.49 in);
otherwise, the zero is not necessary (e.g., *p* = .93).

**GN06:** For numbers over 1,000,
we *must* use the comma to separate groups of three digits
except for page numbers, binary code, serial numbers, temperatures,
acoustic frequencies and degrees of freedom.

**GN07:** We *must not* add apostrophes
when we write a plural of a number (e.g., the 90's).

**GN08:** When one number follows another in a sentence,
we *should* mix numbers with words (e.g., 4 three-year-old dogs),
but, in some cases, the wording may be modified for clarity.

**GN09:** We *must not* use the superscript
for ordinal numbers (e.g., 1^{st}, 3^{rd}, 9^{th}).

**GN10:** When we are presenting values with decimal numbers in a text,
we *must not* give the values of the same variable
with a different number of decimals.

**GN11:** We *must* use words instead of mathematical symbols
(e.g., plus, equal) in narrative texts.

**GN12:** We *must* use spaces between elements in mathematical expressions
to facilitate reading (e.g., a Ã— -b = c;
here, the minus sign is not separated from the *b*
because it represents its negative value).

**GN13:** When we talk about monetary values of millions and billions,
we *should* use the abbreviations M and B, respectively (e.g., $15.4M).
We *must not* express the whole number unless strictly necessary,
for example, for some kind of analysis.