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Punctuate 
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Essential vs. Nonessential Information

Probably the toughest punctuation mark to use is the comma.  While there are actually very few rules, the seemingly infinite variety of situations can make usage tough.

The rule that seems to cause the most grief is the one concerning essential or nonessential information, information often introduced by which, that, or who.  

Essential information must be in the sentence to allow for complete understanding.  If you take it out, the sentence is either terribly confusing or doesn’t work any longer.  This information often helps a reader distinguish one thing among many by describing some pertinent detail. 

Never enclose essential information within either parentheses or commas.

Nonessential information is grammatically unnecessary in a sentence.  While it can be helpful and perhaps nice to know, it's not critical for complete understanding of the sentence.  It often adds a description to a one-of-a-kind item or person, a description that isn’t necessary for the reader to know which one is meant. 

One way of determining if information is essential or not is to leave it out; if the sentence still makes sense, the information isn’t necessary.  Another way to decide is to see if it makes sense to put the information within parentheses; these usually do enclose extra information.  If you can imagine the information within parentheses, it's probably not necessary to the sentence.

By the way, this is sometimes highly subjective.  The writer has to decide if the information is needed or not.

The comma rule is quite easy:  replace each half of the parentheses with a comma when bracketing nonessential information.  Yes, that means you need two commas to enclose the information -- just as the parentheses did.

Nonessential:  The copier (which is in the hall) is broken.
                       The copier, which is in the hall, is broken.

As written, the basic sentence is "The copier is broken."  In other words, the sentence says that there is only one copier and it's broken.  Its location, while helpful to know, is not necessary information.  We don’t have to include any details to help distinguish it from any other if there is no other!   

Note:  The commas are a better choice than parentheses, which are just too big to use often in a document.  Because they do enclose nonessential information and because most of us are curious and will focus on the unexpected, we're apt to read what's in the parentheses first.  So, we'll be reading the stuff the writer doesn't consider important!  Stick with commas, which are unobtrusive.

Essential material is necessary for complete understanding.

As written, this sentence says that there are at least two copiers, and only the one on the second floor is broken.  This information is essential because it specifies the copier you are talking about.  Wouldn't you like to know which one needs fixing without running all through the building testing each one?

Notice, too, the word that; it's the preferred word to use when introducing essential information (if you use anything at all).   You could write:  The copier on the second floor is broken.

Essential:     The red mittens that are on the table are Sam’s.
                    The red mittens on the table are Sam’s.

This says that of all the red mittens you might find around the house, those on the table are Sam's.  Therefore, the information is essential to distinguish Sam's mittens from anyone else's.  

Nonessential:  The red mittens, which are on the table, are Sam’s.

Here you're saying that there is only one pair of red mittens, and they are Sam's.  The fact that they are on the table is helpful, perhaps, but it's not critical.  No matter where they are, they are still Sam's mittens.

Nonessential:  The man, who wore the red hat, looked funny.  (There was only one man.)

Essential:  They left through the gate that was open.  (Any other gates were closed.)
Nonessential:  They left through the gate, which was open.  (One gate only.)

As writers, we must do our best to help our reader understand what we mean.  It’s not fair to say, “But you knew what I meant!” 

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Book of the Month:  Fierce Conversations by Susan Scott.  This might be the best book I’ve ever read and used concerning the art of creating deep and meaningful conversations.