Do commas really matter all that much? you may wonder.
Of course they do! (The previous sentence would have a different meaning if I’d written it as Of course, they do!) You’ve probably seen at least one of the many internet memes comparing “Let’s eat, Grandma!” and “Let’s eat Grandma!” That’s the difference made by using a comma to separate a direct address from the rest of a sentence, and it’s kinda obvious. On the other hand, what about commas and the nonrestrictive phrase? (Ooooooh, scary jargon! *shakes head*)
The following Venn diagram shows what happens when you use commas to make a nonrestrictive phrase (a phrase that can be omitted without changing the basic meaning of the sentence) where one doesn’t belong.
When who get along well with dogs is punctuated as a nonrestrictive phrase, the sentence is saying that cats — all cats, because there’s no restriction — get along well with dogs and small children. Anyone who knows anything about cats knows this is not true (which is why I used this example).
So what happens when we get rid of the commas?
Now the sentence is true… or at least it seems far more plausible. It now says that the cats who do get along well with dogs tend to get along well with small children, but nothing is stated or implied about other cats. (See how the cats who get along with dogs and children are a sub-set of all cats?) This is because who get along well with dogs is now a restrictive phrase, meaning it restricts (limits/narrows down from a broader category) the thing it describes.
This particular comma issue is on my mind today because I recently read an article about a group of people, and a sub-set of that group, but the article was written (punctuated, specifically) in a way that implies everyone in the larger group matches the description of the sub-set. It doesn’t take a lot of imagination (or maybe it does — I certainly don’t grok how average humans think) to see how this could cause a lot of harm for someone if they were assumed to belong to a group they’re not part of, or to have traits they don’t possess, simply because some writer misused a pair of commas.
I used the example about cats getting along with dogs partly because it’s not “political” or the sort of thing anyone would get angry about. If I’d used an example that dealt with a group of humans — those who vote a certain way, perhaps — someone would take offense at what I had to say about punctuation, because they’d assume I was endorsing the incorrectly punctuated version as factually true. Humans are so willfully stupid sometimes… *sigh* “Humans, who eat meat, have more bio-available protein in their diet.” See what a pair of misused commas can do? Obviously not all humans eat meat, but it’s what that sentence says; who eat meat “renames” humans without restricting that to only some humans.