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In most instances, place a comma between two sentences that are joined with a coordinating conjunction--and, but, or, for, nor, so, yet:

  • She was not sure if she had the necessary mathematical abilities to be an engineer, so she pursued a graduate degree in history.
  • He was surrounded by fifty people, yet he felt all alone.

You do not need to place a comma between two independent clauses if they are short and similar in meaning, provided that no misunderstanding will take place, as illustrated in the following example:

  • Some doctors advertise their services but many doctors find this reprehensible.
  • The absence of the comma in this sentence is acceptable; it is not necessary to prevent misreading.

Other comma resources: