Comma help!?

Question

I'm having a hard time figuring out when, exactly, to put a comma before the word "and" in a sentence. Here are a few sentences which will help to illustrate my question:

"The distance between two consecutive pulses of energy in a periodic wave is called the wavelength and it is an important number that characterizes that particular wave."

If I had written this sentence, I would have put a comma before the "and." Is this incorrect? If so, why?

Here's another sentence I underlined:

"They are sometimes called 'eye safe' lasers, and are not considered hazardous even if the output beam is directed into the eye's pupil for long periods of times."

This sentence has a comma before the "and," but I'm not really sure why.

Again, my question is this: when should I put a comma before the word "and" in a sentence?

Answer 1

"The distance between two consecutive pulses of energy in a periodic wave is called the wavelength, and it is an important number that characterizes that particular wave."

Yes: This sentence needs a comma before the "and." You need a comma between two main clauses (i.e., parts that could stand alone as sentences.

"They are sometimes called 'eye safe' lasers and are not considered hazardous even if the output beam is directed into the eye's pupil for long periods of times."

This one does not need a comma because "and" separates compound predicates rather than main clauses.

Complete explanation here:
http://owl.english.purdue.edu/handouts/grammar/g_comma.html

If this or any other answer to your question helps you resolve this issue, please select a "best answer." This motivates people to help you and rewards their research in your behalf.

Cheers,
Bruce

Answer 2

"The distance between two consecutive pulses of energy in a periodic wave is called the wavelength, and it is an important number that characterizes that particular wave."

If I had written this sentence, I would have put a comma before the "and." Is this incorrect? If so, why? YES THIS IS CORRECT

Here's another sentence I underlined:

"They are sometimes called 'eye safe' lasers, and are not considered hazardous even if the output beam is directed into the eye's pupil for long periods of times."

This sentence has a comma before the "and," but I'm not really sure why. CORRECT

Again, my question is this: when should I put a comma before the word "and" in a sentence?

Answer 3

The correct ways are:

The distance between two consecutive pulses of energy in a periodic wave is called the wavelength, and it is an important number that characterizes that particular wave.

They are sometimes called 'eye safe' lasers and are not considered hazardous, even if the output beam is directed into the eye's pupil for long periods of time.

You put a comma before the word and (or but) only if you are separating two possible sentences (there is a more precise rule but I cannot remember it). For example:

I love to eat, but I do not enjoy cooking.

I love to eat but do not enjoy cooking.

HTH.

Answer 4

When you are connecting two thoughts that could be complete sentences on their own, it is appropriate to use a comma.

In this case, both sentences should have the comma you are referring to.

Just think of it as giving the brain a breath to pause and digest the first complete thought before getting into the second. Makes it easier to read.

Answer 5

The grammar rule is related to the use of commas with coordinating conjunctions:

FANBOYS is a handy mnemonic device for remembering the coordinating conjunctions: For, And, Nor, But, Or, Yet, So.
COMPLETE THOUGHT = is a group of words (simple sentence) with a subject and verb. COMPLETE THOUGHT CHARACTERISTIC: It can stand alone as a sentence.

RULE:
► You should always have a comma before FANBOYS that join two complete thoughts (with two subjects and two verbs).
<== complete thought ==> FANBOYS <== complete thought ==>
Mary read the book, and she saw the movie.

► If you do not have two subjects and two verbs separated by the FANBOYS, you do not need to insert the comma before the FANBOYS. In other words, if the second grouping of words isn’t a complete thought, don’t use a comma:
<== complete thought ==> FANBOYS <== NOT complete thought ==>
My hamster loved to play and often ate marshmallows
John traveled to France and then to Spain

► Exception: When the FANBOYS join two complete thoughts, but the second thought is short.
<== complete thought ==> FANBOYS <== complete thought ==>
She invited him to the party and he accepted

Some Example:
Marion read the paper and didn't agree with its thesis
(complete clause) FANBOYS (incomplete clause - no subject)

Marion read the paper, and SHE didn't agree with its thesis
(complete clause) comma FANBOYS (complete clause)

Mary read the book, and she saw the movie. (USE COMMA)
Mary read the book and saw the movie. (DON'T USE COMMA)