I was writing along for one of the "real jobs" (that is, the ones that pay), and in a response to a submission, wrote a sentence structured like this, "As I said earlier, I grew up near Richmond, and recognized many of the monuments portrayed in the photographs."
Now we all know the rule. To distinguish between the two independent clauses in a compound sentence, we use a comma plus a conjunction (, and). But there aren't two independent clauses. There's a dependent clause: As I said earlier, appropriately followed by a comma. (No, I won't argue the alternative or go into the very few exceptions here. I read too many student papers where there's no distinction between the introductory element and the main clause. Trust me. The reader needs it, and it was only the AP's need to save ink for newspapers that made this at all arguable. If you have read a thousand student sentences like this where there are no commas to distinguish sentence elements you would agree that a comma is usually necessary in complex sentences.)
There's a main clause, with the sentence subject and predicate: I grew up near Richmond
And there is a second predicate with a direct object: recognized many of the monuments portrayed in the photographs.
The need for the conjunction is clear as we're linking two predicates, and the second predicate (recognized) with the sentence subject (I). But the comma before the "and"-- what's that about? Why did I immediately put that in? Ordinarily I wouldn't. Ordinarily, with one subject and two predicate sentences, I would just use the conjunction. So why did I use the comma? (I could, of course, just have made a mistake, but I don't usually make that kind of mistake -- years of editorial work have trained me in punctuating, though it didn't beat the dash-addiction out of me.) When I do something like that, I go back and try to figure out why I did it. I trust my instinct, but question it too. (Hey! I just did it again-- comma before the conjunction before a second predicate! Why? I think because the second is a contradiction of the first, so I signify that with the comma-- "... here comes something different.")
Anyway, back to the original sentence. (And the point of this is to say that the rules are important because they're based on the logic of sentencing, and when you understand the logic, you can tell when this particular sentence needs a non-rule treatment because it's doing something that is outside that logic.)
The kicker here isn't actually the subject (I) or the double-predicate. It's that introductory element (As I said earlier). What exactly did I say earlier?
What I said earlier in the letter was (in response to the setting of the book) that I had grown up near Richmond. I was trying to establish commonality and also present the reality that I was perhaps more able to appreciate an aspect of the book (the photos) than another reader.
But... what I didn't say earlier was this:
recognized many of the monuments portrayed in the photographs.
That's new. I said "main clause with predicate 1" before, but not "trailing phrase with predicate 2" before.
Without the comma, I would be saying that I'd said all that before, and the reader would be looking back to the first paragraph and wondering what had happened to the mention of the recognition.
The comma says, "What's before me is separate in some meaningful way from what's after me." I don't have to necessarily specify what the difference in meaning is, but the comma is a recognition of that so that the reader doesn't have to wonder. We can trust the reader to sense if not completely consciously understand that there is a distinction between this:
As I said earlier, I grew up near Richmond and recognized many of the monuments portrayed in the photographs. (That is, I said all of this earlier.)
As I said earlier, I grew up near Richmond, and recognized many of the monuments portrayed in the photographs. (I said only the Richmond part earlier.)
Now I could also have done it the standard compound (two clause) way:
As I said earlier, I grew up near Richmond, and I recognized many of the monuments portrayed in the photographs. (That is, the trailing phrase is now a clause with its own subject.)
That would have been just fine, except three I's in one sentence might seem repetitive, not to mention egotistical. So precision of meaning gave way to reverse-vanity, I guess.
Point is, as usual-- Know the rule. Understand the underlying logic. Intuit exceptions. Second-guess your intuition. Make a conscious ruling.
Do all that, and I'll even shut up about exceptions to the rule about commas and intro elements... but you have to add, "Explain your rationale" for that one. :)
Punctuation is part of the construction of meaning in a sentence and paragraph.