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by Naomi Nemtzow

When we speak, we emphasize certain words and pause after others. We also vary the intonation and expression in our voices, and we combine facial expressions and body language with our words.

All of these characteristics of spoken language, which we often use without even thinking, help listeners to understand what our words mean. When we write, the reader does not have the benefit of hearing our voices or seeing our gestures. That is why punctuation marks are so important; they help give meaning to our words.

Period [ . ]

Use a period at the end of a sentence. That sounds easy enough. What is not always easy is knowing when we have reached the end of a sentence!

Comma [ , ]

Some common uses for commas are:

  1. To set off an introductory dependent clause before the main independent clause in a sentence.
  2. Example: Because there was trouble with the equipment, the experiment was postponed.

  3. To set off parenthetic words and phrases:
  4. Example: Some of the materials were too light. For example, foam was inappropriate for the container.

    Be aware that if the parenthetic phrase is in the middle of a sentence, you need two commas: one before the phrase and one after:

    Example: Some of the components, many of which were silver, were too light.

  5. To separate three or more items in a series:
  6. Example: The microphone was made from a Styrofoam cup, aluminum foil, plastic wrap, and piezoelectric film.

  7. To separate coordinate adjectives i.e., adjectives that could logically be linked by the conjunction and:
  8. Example: We added a large, crumpled piece of aluminum foil to the container.

  9. A common error is to link two independent clauses with a comma. This is called a comma splice.
  10. Example: The protective containers were then tested, they were dropped from a height of 50 feet.

    To correct this error, place a period or a semicolon between the two independent clauses, or if appropriate, add a conjunction such as and between the clauses.

    Example: The protective containers were then tested; they were dropped from a height of 50 feet.
    Or: The protective containers were then tested, and they were dropped from a height of 50 feet.

Semicolon [ ; ]

Semicolons are useful when you want to connect independent clauses tightly. Two clauses linked by semicolons are more closely connected than two short, but separate sentences. However, do not overuse the semicolon; save it for those times when you really need it.

Example: The protective containers were tested; they were dropped from a height of 50 feet.

Semicolons are also used to separate items in a list when the items themselves are punctuated. When used like this, they are sometimes called super-commas:

Example: Members of the panel included Janie Wong, representing NYU Polytechnic School of Engineering; George Jones, representing Cooper Union; and Allison McNeil, representing Rensselaer Polytechnic Institute.

Colon [ : ]

Use a colon:

  1. To introduce a list.
  2. Example: The materials available for making the microphone were: Styrofoam cups, plastic cups, aluminum foil, plastic wrap, and piezoelectric film.

  3. To introduce a restatement or an explanation. If you can substitute the word namely for the colon, you are using it correctly. You may capitalize the first word after the colon.
  4. Example: Thomas Edison had a careful procedure for keeping his laboratory notebooks: He made sure all entries were signed, dated, and properly witnessed.

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