If it’s not the comma, it’s the semicolon that causes the conscientious writer so much trouble. Would you agree, for example, that the semicolon is used correctly in this sentence: The investigators determined that the pilots were not at fault; maintaining that the evidence of structural failure was conclusive. The art of punctuation is a subtle one, and often there is no one right answer. In this example, however, there is: it’s incorrect. But why?
The secret to the semicolon can be seen in its shape: it’s a period stacked directly over a comma, and it has been designed that way (whether by intention or accident) to create a mark that signals to the reader to slow and distinguish slightly two sections of a sentence (the function of the comma), but not so much that what follows it could begin an entirely new idea (the function of the period). The semicolon, that is to say, is stronger than a comma, but not quite as strong as a period; it marks off, but doesn’t cut off, closely related ideas (as the semicolon in this sentence illustrates).
And who is to decide how closely the ideas of a sentence are to be related? The writer, of course, but that decision must be made within the technical requirements of grammar and sentence structure. In our example, my guess is that the writer chose the semicolon instead of a comma in order to emphasize the dramatic idea that follows it (a justification one often hears in matters of punctuation). Isolating a portion of a sentence will certainly emphasize its meaning, but here the dramatic wish has clashed with the standards of the grammar.
If we look closely at the nine words following the semicolon, we see that they begin with the participle maintaining. Participles are adjectives, which means they have to modify a noun or pronoun somewhere else in the sentence. The eight words following the participle maintaining constitute a noun clause which tells the reader what exactly was being maintained. But who was maintaining that the evidence of structural failure was conclusive? The investigators, of course. The noun investigators, however, appears in that part of the sentence on the other side of the semicolon wall (built up by the strength of the period in the semicolon), and is therefore beyond the easy reach of the participle to modify. The ten words up to the semicolon constitute an independent clause; the nine words after it comprise a subordinate element of participle and noun clause. This unbalances the sentence and rules out the semicolon because semicolons most often coordinate independent, not subordinate, elements in a sentence.
So how should this sentence be punctuated? Simply with a comma instead of that semicolon: The investigators determined that the pilots were not at fault, maintaining that the evidence of structural failure was conclusive. Now the magnetism of the participle, so to speak, can without any obstruction reach to its noun investigators. And as for the dramatic isolation the writer was probably originally after, the comma sections off this second half of the sentence as a distinct portion just as effectively, and let’s the power of the meaning impress itself on the reader’s mind without the grammatical irregularity.
But before we conclude, let’s make a little more trouble and ask what it would take to preserve the semicolon in our example. If fact, not much. All we have to do is change the participle maintaining into a finite verb with its own subject. That would create an independent clause in its own right, and that independent element would balance nicely with the independent elements already in place before the semicolon: The investigators determined that the pilots were not at fault; they maintained that the evidence of structural failure was conclusive. We have then produced a balanced sentence, and that is what the semicolon most often does.
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