Using apostrophes to show possession

You use an apostrophe to show that a thing or person belongs or relates to someone or something: instead of saying the party of Ben or the weather of yesterday, you can write Ben’s party and yesterday’s weather.

  • EXAMPLE:  We met at Ben’s party.   
  • EXAMPLE: Yesterday’s weather was dreadful.

master’s degree, bachelor's degree   

Use an apostrophe to spell ‘ master’s degree’.  The ‘s’ in ‘master’s’  indicates a possessive (the degree of a master), not a plural.  

  • EXAMPLE: .. she is doing a master’s degree in engineering

If you are writing of a specific degree, you should capitalise 'master' and avoid creating a possessive.

  • EXAMPLE: ... she is doing the Master of Science (MSc) in Mechanical Engineering at the University of Twente  


The same rules apply to a ‘bachelor’s degree’.

  • EXAMPLE: He hopes to complete his bachelor’s in record time.
  • EXAMPLE: The Bachelor of Science (BSc) degree at the University of Twente has an excellent reputation.


The title of these degree qualifications is 'Master of Science' and the general title is 'master’s degrees', so a student of a master’s degree is a master’s student

  • EXAMPLE:  he tutors a master's student every week

Personal names that end in –s

With personal names that end in -s: add an apostrophe plus s when you would naturally pronounce an extra 's' if you said the word out loud:

  • EXAMPLE: He joined Charles’s army in 1642.        
  • EXAMPLE: Dickens's novels provide a wonderful insight into Victorian England.

Note that there are some exceptions to this rule, especially in names of places or organisations, and it is sometimes a question of personal choice and judgement:

  • EXAMPLE: St Thomas’ Hospital
  • EXAMPLE: (some writers prefer) Charles' book (ie. without the apostrophe 's')

Plural nouns that end in –s

With a plural noun that already ends in -s: add an apostrophe after the s:

  • EXAMPLE:  The mansion was converted into a girls’ school.                    
  • EXAMPLE: The work is due to start in two weeks’ time.

Plural nouns that do not end in -s

With a plural noun that doesn’t end in –s: add an apostrophe plus s

  • EXAMPLE: The children’s father came round to see me.   
  • EXAMPLE: He employs 14 people at his men’s clothing store.

Possessive pronouns do not need apostrophes

The only cases in which you do not need an apostrophe to show belonging is in the group of words called possessive pronouns - these are the words: his, hers, ours, yours, theirs - (meaning ‘belonging to him, her, us, you, or them’) - and with the possessive determiners. 

  • EXAMPLES:  It was his party.  It is their house. 

Possessive use of ‘its’.  It’s or its?

These two words can cause a lot of confusion: many people are uncertain about whether or not to use an apostrophe.

Rule: ‘its’ (without an apostrophe) means ‘belonging to it’:

  • EXAMPLE: The dog wagged its tail.


Rule: it’s (with an apostrophe) means ‘it is’ or ‘it has’:

  • EXAMPLE:  It’s (it has) been a long day.  
  • EXAMPLE: It’s (it is) a comfortable car and it’s got some great gadgets.                                                                                                            

Apostrophes showing omission (contractions)

An apostrophe can be used to show that letters or numbers have been omitted.

  • EXAMPLES: 
  • I’m - short for I am
    he’ll - short for he will
    she’d – short for she had or she would
    pick ’n’ mix - short for pick and mix

It also shows that numbers have been omitted, especially in dates.

EXAMPLE: The Berlin Wall came down in the autumn of ’89 (short for 1989).

Common mistakes and some exceptions

The general rule is that you should not use an apostrophe to form the plurals of nouns, abbreviations, or dates made up of numbers: just add -s (or -es, if the noun in question forms its plural with -es).

  • EXAMPLE:  The cost of the trip is 570 euros.     AVOID: euro’s
  • EXAMPLE:  Traditional Italian pizzas are thin and crisp.     AVOID: pizza’s
  • EXAMPLE:  Local MPs are divided on this issue.     AVOID: MP’s 
  • EXAMPLE:  The situation was different in the 1990s.     AVOID: 1990’s


two exceptions

There are one or two cases in which it is acceptable to use an apostrophe to form a plural, purely for the sake of clarity. 

1. You can use an apostrophe to show the plurals of single letters

  • EXAMPLE:  I've dotted the i's and crossed the t's.
  • EXAMPLE:  Find all the p's in appear.

2. You can use an apostrophe to show the plurals of single numbers

  • EXAMPLE: Find all the number 7's.


Reference

Oxford Living Dictionaries. 'punctuation/apostrophe'. Retrieved from https://en.oxforddictionaries.com/punctuation/apostrophe.