There are fewer rules for commas than for any other punctuation. Authors tend to overuse them, and don’t realize it. That preceding sentence is an example of a comma that serves no purpose being there. It isn’t a compound sentence. A compound sentence is two complete sentences linked by a conjunction, and “don’t realize it” isn’t a complete sentence. In fact, the two parts of the sentence share a subject (“Authors …”) “Authors tend to overuse them and don’t realize it” is more correct. “Authors tend to overuse them without realizing it” would be an improvement over either sentence. Why, I don’t know, but that error has begun to arise everywhere, including in books that I would otherwise have judged were very well copyedited.
Sometimes, when a compound sentence is especially convoluted, like this one, and it takes some effort to to keep track of which modifiers link to which subject, or verb, or object, yet you don’t want to interfere too heavily because the author clearly has a good grasp of his own syntax, you can happily let the extraneous comma go through, and think of it as a breather comma. The comma after “through” – that’s a breather comma.
If you can lift a clause out of sentence without distorting the meaning of what’s left, you need to plant a comma at either end. “Greyhounds, which I have always owned, are fast runners.” When you take out the clause, greyhounds are still fast runners. Nice of you to have adopted so many greyhounds, but the meat of the sentence is that they are fast runners, and you don’t distort that information by taking out the clause. So, a comma at either end of that clause (or a dash at either end, though that would be a bit of overkill). “Greyhounds that have three legs can still run fast.” If you take out the clause (“that have three legs”), you’re distorting what’s meant to be the point of the sentence, which is an observation about three-legged greyhounds. So, no commas at either end of that clause.
By the way, I should add something about parentheticals. In that sentence, By the way is parenthetical, in the sense that you’re preparing the reader for a slight change of direction. Put another way, you’re engaging in verbal throat-clearing. In other words, you’re conveying that the next point you’re making is going to be some sort of clarification of the preceding point. You’ll find that written English is saturated with parentheticals, which serve something like fingerposts through the verdant countryside of your prose. And thank heaven they do. And they always take a comma.
An observation and a complaint about conjunctions
There is no reason on God’s earth why you can’t start a sentence with a conjunction. And most authors occasionally do. But some authors absolutely howl when you try to allow that. I won’t go into why opening conjunctions are acceptable – it just seems obvious that they are. Just don’t do it all the time, because it gets monotonous. Do it too often and the reader will take it for a cheesy rhetorical device, which it is, when overdone.
None of which means that you should ever start a sentence with a double-conjunction. And yet some authors do. And yet is one of my pet peeves. The yet covers what you’re trying to say, the and is superfluous. It amazes me how often authors do that, and override the deletion when I make it.
An observation about myself
My wife has finished reading the first few posts of this blog and tells me I sound just like myself. That’s gratifying. A writer who sounds just like himself is doing something right.