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In the sentence "Their work involves commercial solar heating systems," should there be a comma after the word "commercial"? These are solar heating systems for commercial use as opposed to private homes, so I see the entire phrase as descriptive of the systems and therefore not needing a comma.

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No, the comma is not needed.

Although commas are sometimes used to separate adjectives, they are used in that way only in certain circumstances:

  1. Use commas to separate two or more coordinate adjectives that describe the same noun. Be sure never to add an extra comma between the final adjective and the noun itself or to use commas with non-coordinate adjectives.

Coordinate adjectives are adjectives with equal ("co"-ordinate) status in describing the noun; neither adjective is subordinate to the other. You can decide if two adjectives in a row are coordinate by asking the following questions:

  • Does the sentence make sense if the adjectives are written in reverse order?
  • Does the sentence make sense if the adjectives are written with and between them?
    If you answer yes to these questions, then the adjectives are coordinate and should be separated by a comma.

  • Because the adjectives in your sentence aren't coordinate(check them out), no comma is needed.

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    Actually, commas can be used between two adjectives if both adjectives modify the same noun: "He was a sweaty, grimy mess" contains two coordinate adjectives and therefore uses a comma. link – ect Sep 9 '11 at 23:36
    I've heard that, in cases when "and" would sound okay, you can substitute a comma for the conjunction. (But that was just a shorthand way of teaching high schoolers to game the ACT.) – ect Sep 9 '11 at 23:40

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