I'm wondering what the proper punctuation is when using "as is" to make a comparison.

Example: "Venus and Mars are planets, as is Earth when referring to the whole world."

I've noticed in this answer regarding a different use of "as is" the poster uses a comma, and in this question regarding non-adverbial uses of as-is the poster also includes a comma before "as is".

This was my first instinct, but when looking back over the sentence, I wasn't sure anymore. Is the second half of the sentence parenthetical (and thus belong in parentheses), or if the comma is correct, then why? Are both sides of the sentence independent clauses? If they are not independent clauses then should there be any punctuation at all?

  • Note that one can argue that "when referring to the whole world" is also parenthetical (to "as is Earth"). Thus there should arguably be two commas. This can be confirmed by noting that there would typically be two pauses when speaking the sentence. – Hot Licks Apr 13 '16 at 19:47
  • I assumed that since "Earth's" status as a planet is conditional based on the information that follows (earth the substance is not a planet), the comma would be incorrect. But maybe that's irrelevant? – user170207 Apr 13 '16 at 20:20

In this example, "as is Earth ... " is a subordinate clause. It is introduced by a subordinate conjunction, as. "Venus and Mars are planets" is an independent clause--it stands on its own. The fact that Earth is also a planet does not affect the truth or the meaning of that. Everything after the comma is, in effect, a "parenthetical" comment, and the comma here is correct. (Parentheses would also work, as would a dash--anything to separate the subordinate statement is OK.)

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  • Nice. Subordinate clause it is! – user170207 Apr 13 '16 at 20:12

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