Use of comma and its effect on the meaning [closed]

Solution 1:

According to the convention of traditional grammar, a comma should be used to present nonessential information—and if there is no comma, the same information is considered essential.

In the case of your two sentences:

1. My sister, Grace, lives in Toronto.

The use of the commas indicates that you only have one sister. Providing the name isn't necessary because the essential meaning of the sentence would be understood if you hadn't provided her name:

My sister lives in Toronto.

The use of the commas is the equivalent of this:

My sister (whose name happens to be Grace) lives in Toronto.

2. My sister Grace lives in Toronto.

Here, the lack of commas implies that you have at least two sisters. You are providing the name in order to give essential information that restricts the subject of the sentence to a particular sister.

In other words, it could be followed by something about another sister:

My sister Audrey lives in Montreal.

However, if you do have more than one sister, and you phrase the sentence differently, you certainly don't need to provide a name:

One of my sisters lives in Toronto.

I've had a couple of comments mention another use of commas. I had been aware of it, but I didn't mention it because I didn't consider it part of a direct contrast between the two.

In short, if you know somebody named Grace, and you are addressing them explicitly by name, their name could be used as in interjection in the middle of the sentence.

In other words:

Grace, my sister lives in Toronto.
→ My sister, Grace, lives in Toronto.

However, you could also say this (for the same reasons):

My sister Anne, Grace, lives in Toronto.

Here, you have more than one sister, and the one you're talking about is Anne. At the same time, you are making an interjection to address Grace, to whom you are talking.

The interpretation of this is still feasible, but I think it would be at the point where it would be too confusing to let stand as written—even if you knew that the person being spoken to was Grace.