No. 1 rule of AP style
Let’s focus on everyone’s favorite number: No. 1.
Yup, it’s written just like that, according to the Associated Press style guide. For example, The No. 1 box office hit this weekend was “X-Men: First Class.”
Also: No. 2, No. 3, etc.
Although we use a numeral for 100, spell it out in a casual reference: About a hundred or so. Also in a casual reference: The store is a quarter mile or so. There is a half of watermelon on the table. About a third of the class was paying attention.
The Associated Press style guide says to use this method if precision is not intended.
Where precision is intended, spell out fractions less than one: one-half, one-third, one-forth, etc. However, use a numeral for fractions larger than one, and leave a space between the whole number and the fraction: 1 1/2, 1 2/3, etc.
Use a Roman numeral when referring to World War I. You can use WWI on second reference. Also: WWII.
Also common No.1 phrases:
The article is one-sided. He was a one-time writer. He wrote just one time.
Remember: The Supreme Court uses one person, one vote (NOT one man, one vote). Use hyphens with the adjective form:
He cited the one-person, one-vote rule. But: He supports the rule of one person, one vote.
When referring to amendments in the Constitution, spell out First through Ninth: the First Amendment, the 16th Amendment.
Military No. 1 terms: Air Force One, 1st Lt., 1st Division, 1st Fleet
For street names, spell out first through ninth: I live on First Avenue, but she lives on 13th Street.
When referring to amounts of money, always use numerals, but starting with a million, spell it out like this: 1 million, 1.5 billion, 1.8 trillion.
Times: An hour before 1 p.m. is noon. An hour before 1 a.m. is midnight.