All cats (and some dogs) like fish.

A parenthetical expression is a word or words added to a sentence without changing the meaning or grammar of the original sentence. Parenthetical expressions give extra information but are not essential. You can add and remove a parenthetical and the sentence works just the same.

Some information in a sentence is essential to its meaning, and some information may be less important or "nonessential":

Essential:  The person taking money at the door asked for a form of identification.

Nonessential:  The police, hoping to save time, took the expressway.

When nonessential information is added parenthetically to a sentence, it is usually separated from the main sentence by commas or other punctuation. In fact there are three types of punctuation that can separate parenthetical expressions:

  • commas  , .......,
  • round brackets (.......)
  • long dashes —.......—
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Commas are the usual form of punctuation for parenthetics. Remember, if the parenthetic appears in the middle of the sentence there must be TWO commas, one before and one after.

Brackets are the next most common form of punctuation for parenthetics. Bracketed parenthetics can only appear in the middle of the sentence, and there must always be TWO brackets.

Dashes for parenthetics are much less common. If the parenthetic appears in the middle of the sentence, there must be TWO dashes, one before and one after.

Look at these examples of parenthetical expressions:

  • Some foods, sugar for example, are not good for us.
  • Timothy , who lives near Stonehenge, goes to church regularly.
  • Tara , although she comes from a hot climate, hates hot weather.
  • Anthony, however,  decided not to go.
  • The planet closest to the sun (ie Mercury) has the most extreme temperature variations.
  • The 70th anniversary of the D-Day landings (6 June 1944) was attended by many world leaders.
  • The cheetah—the world's fastest land animal—is native to Africa.
  • If they didn't understand you—a qualified teacher—how will they ever understand me?

Note that in all the above cases, where the parenthetic is in the middle of the sentence, there must be a PAIR of punctuation marks—an opening mark and a closing mark. However, when a parenthetic is at the beginning or end of a sentence, we can use a single comma or a single dash. It is NOT possible to use a single bracket. Look at these examples:

  • Well,  how are you going to explain yourself now?
  • You should never drink and drive, of course.
  • John had not been drinking—or so he says.
Parenthetical expressions add information but can easily be removed without destroying the meaning or grammar of the main sentence.

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