What is the effect of apostrophes?

Apostrophes in the English language are used to denote
letters omitted or to denote possession.  We use the apostrophe in writing contractions
so that "do not" becomes "don't".  The apostrophe after the "n" indicates that a letter
is missing.  In this case the letter is "o".  Similarly, when we write abbreviations
such as "ass't" for "assistant", the apostrophe again indicates that letters are
omiitted.  In this instance there are several letters, "istan", that are left out.  The
apostrophe is also used to note possession, or ownership.  In singular possessive nouns,
the apostrophe is put before the "s" usually, as in "girl's" such as might be found in
this sentence: The girl's dress was blue.  In plural possessive
nouns, for nouns that form their plural by adding "s" or "es", the apostrophe comes
after the "s" as in the word "girls'" in the following sentence:  The girls'
dresses were blue.  In nouns that form their plurals in an irregular way,
such as "woman" that becomes "women" when pluralized, the apostrophe comes before the
"s" as in "women's" such as in this sentence: The women's dresses were
blue.