What's the rationale for "this," instead of "this", (comma outside the double quote)

The comma goes inside the quotes in dialogue, and it goes outside the quotes in all other situations:

“I told you to do this,” said Mr. X.

“This”, “these”, “that”, and “those” are all examples of demonstratives.

“There are peculiar typographical reasons why the period and comma go inside the quotation mark in the United States. The following explanation comes from the “Frequently Asked Questions” file of alt.english.usage: “In the days when printing used raised bits of metal, ”.” and ”,” were the most delicate, and were in danger of damage (the face of the piece of type might break off from the body, or be bent or dented from above) if they had a ’”’ on one side and a blank space on the other. Hence the convention arose of always using ’.”’ and ’,”’ rather than ’”.’ and ’”,’, regardless of logic.” This seems to be an argument to return to something more logical, but there is little impetus to do so within the United States.”

Source

Yes, it is corrrect. In standard American usage, the comma and period go inside the quotes. The only exception is where the quoted content is a literal string (such as a computer password) that does not include the mark of punctuation.

Colons and semicolons go outside, and question marks and exclamation points go either inside or outside, depending on whether they are included in the quoted material.

American and British styles differ with respect to this and various other conventions.

How is a declarative sentence that ends with a quoted question punctuated?
Ratboy asked “how is a declarative sentence that ends with a quoted question punctuated?“__

@ratboy : No further punctuation after the question mark and the ending quotation mark is needed. And you need a comma after the word asked in your sentence.

Ratboy asked, “How is a declarative sentence that ends with a quoted question punctuated?“

Did Ratboy say, “I adore correct punctuation”?

@hawaii_jake: thanks for the comm[a]ent..
Did Jeruba really ask, “Did Ratboy say, ‘I adore correct punctuation’?”?

I don’t think so, @ratboy, although there’s a certain logic to that. But if I’d written

   “I heard Ratboy say, ‘I adore correct punctuation.’”

I would have used only one period, not two. Likewise, I would suggest that you write

   Did Jeruba really ask, “Did Ratboy say, ‘I adore correct punctuation’?”

But first I would try really hard to persuade you to recast that suffering sentence.