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:
- Use a Comma After Conjunctive Adverbs and Transitional Phrases at the Beginnings of Sentences
- Use Commas After Introductory Subordinate Clauses
- Use Commas Around Nonrestrictive Parenthetical Elements
- Use Commas Before Nonrestrictive Adverbial Phrases or Clauses at the Ends of Sentences
- Use Commas to Separate Adjacent Parallel Elements