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not only ... but also

The word that follows not only should grammatically parallel the word that follows but also.
            
Incorrect:     He not only visited Laos but also Thailand.
            Correct:         He visited not only Laos but also Thailand (or but Thailand as well).
                                      He not only visited Laos but also traveled through Thailand.

Do not use a comma before but also unless it connects two independent clauses or the sentence is unusually long.

But or but also?    When not only means "in part," but should be joined by a term such as also, as well, or too that indicates inclusion and addition:
            We saw not only an executive but also the chief executive.  (Two executives.)

The but should stand alone only when the term that follows it intensifies the thought that follows not only:
            
He is not only an executive but the chief executive.