1. Use the apostrophe and s ( 's ) to form the possessive singular of most nouns not ending in - s.
The manager's office.
2. Use the apostrophe alone ( ' ) to form the possessive plural of common nouns ending in - s.
The executives' board meeting.
3. Use the apostrophe and s ( 's ) to form the possessive plural of nouns not ending in - s.
The children's toys.
4. Use the apostrophe and s ( 's ) to form the possessive singular of proper nouns of one syllable ending in - s, - x, - ch, or - sh.
Mr. Santos' s car.
Miss Felix's bag.
Atty. Leech's case.
Mr. Marsh's office.
The foregoing rule is not absolute. There is a growing tendency today to shorten these forms and speak of Mr. Santos' car, Miss Cox' bag, etc. Most authorities, however, still consider the use of apostrophe and ( 's ) preferable.
5. Use the apostrophe alone or with the s to form the possessive singular of proper nouns of two or more syllables ending in - s, - x, - ch, or - sh.
Mr. Santos' s ( or Santos' ) car.
Miss Reyes's ( or Miss Reyes' ) bag.
Atty. Felix's ( or Atty. Felix' ) case.
Ease, euphony, and clearness are the sole factors in determining the form to use except where good usage prescribes a particular form.
6. Use the apostrophe alone ( ' ) to form the possessive plural of proper nouns.
the Reyeses' orders
the Santos' purchases
7. Add the apostrophe plus s ( 's ) to the last word to form the possessive of compound nouns.
Mr. Reyes's mother-in-law's car; his father-in -law's house.
8. Add the apostrophe plus s to the last name in the series only, to indicate the joint possession; to indicate individual possession, use the apostrophe after each name.
Ravelo and Martires's company (joint ownership).
Ravelo's and Marires's companies are across the street from each other (individual ownership).
9. Use the apostrophe plus s after indefinite or impersonal pronouns to form the possessive.
Someone's records; anybody's customer.
Never use the apostrophe with any one of the possessive pronouns: his, hers, yours, its, ours, theirs, whose.
10. Use the apostrophe in a contracted word to indicate the omission of a letter, letters, or figures.
It's, we're, I'd, o'clock, wasn't
The apostrophe is used to indicate the omission of a part of a date.
Make sure that the apostrophe is placed properly in contracted words. In negative contractions for example, put the apostrophe between the n. and the t, as: isn't, can't, weren't, couldn't, and so on.
11. Use the apostrophe to indicate the plural of letters, figures, and signs.
Write you m's and n's more legibly.
There are four 8's in that final figure.
12. To form the plural of words simply referred to as words, add s only. If the plural form is not clear, add an apostrophe plus s.
There are too many ands and sos in your letters.
Your a's and an's are often misused.
When to avoid the use of apostrophe.
1. Avoid using the possessive case with inanimate objects. It is better to use an of-phrase.
the handle of the cabinet (Not: the cabinet's handle)
the radiator of the car (Not: the car's radiator)
Certain idiomatic expressions indicating time, measure, or personification may take the apostrophe plus s.
a week's salary; a yard's length ; for pity's sake.
Acceptable also as a result of common usage:
the city's mayor; the town's council; Rizal's governor.
2. The apostrophe is often omitted in the titles or names of companies and associations.
Filipino Merchants Insurance Company
Farmers Co-operative Bank