The noun is "decoration."
"odd" is an adjective modifying the noun (odd decoration).
Isn't "room" being used as an adjective here too? What type of decoration? A room decoration.
So when you have a list of adjectives, don't you put a comma after each one except the one to the left of the noun?
odd, room decoration
With due respect, everything you said is correct, except your conclusion.
I think the correct conclusion is you need a comma unless the royal order of adjectives can be applied, in which case you have nothing except a space between the adjectives.
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I haven't figured out this out to my satisfaction yet, but for the purposes of discussion, these are the processes I would use to punctuate a long string of words modifying a noun.
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1. The first step is to find all "compound adjectives" and join them with hyphen(s). [There is an exception for adverbs ending in -ly, but ignore that for the moment. The decision about whether to remove the hyphen will come later.] I think the test is what happens if you delete the final word.
* If the result is complete nonsense, you have a compound adjective, and you need to hyphenate it to the preceding word. Your final word is now a newly hyphenated one. You may need to test if deleting that results in complete nonsense too, in which case you need to find and hyphenate a compound adjective of three or more words.
* If the result still makes sense, and only some detail of meaning has been lost, then you have a single-word modifier.
* If the result still makes sense, but the meaning has been substantially changed, then you have a compound adjective which is potentially ambiguous. You need to hyphenate that, and you cannot remove the hyphen later on even in situations where hyphens are usually not used. An example is 'most skilled workers' when your intended meaning is the workers who are most skilled. The result are deleting 'skilled', 'most workers', still makes sense. That should alert you to the potential ambiguity. Readers might interpret either 'the most-skilled of the workers' or 'most of the skilled-workers'.
- After this step you will have separated all of the adjectives into distinct concepts, some of which may be hyphenated.
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2. The next step is optional, and rarely used.
You may always draw attention to one of your list of (compound) adjectives by bringing it to the front of the list with a comma after it. An example would be if asking someone to select the red ball from a collection of big rubber balls. You would write that as 'red, big rubber ball'.
Note that if you were only identifying one ball you would write 'big red rubber ball' – with no commas. You would not use commas because placing the three adjectives in any other order sounds "unnatural".
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3. The next step is looking for the (compound) adjectives that cannot be moved further away from the noun without sounding unnatural, in which case you do not use a comma before them. For the example of 'big red rubber ball':
* 'red, rubber ball' sounds natural, but 'rubber, red ball' does not. You may then think of 'rubber ball' as being a noun phrase being modified by 'red', so it does not need a comma after the adjective which precedes it.
* Then test 'big, red rubber ball'. It sounds natural, but 'red, big rubber ball' does not. You may then think of 'red rubber ball' as being a noun phrase being modified by 'big', so it does not need a comma after the adjective which precedes it.
That must end once you strike any pair for which the order can be reversed without changing meaning. For example, 'fast, furious chase' means the same as 'furious, fast chase', so whichever order you choose there must be a comma separating them, and commas must separate all other before them too.
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4. Finally hyphens may be removed from some compound adjectives (unless you've decided against that because of potential ambiguity). That applies when when the first word is an adverb ending in -ly. They usually should have no hyphen. The same applies to some common quantifying adverbs not ending in -ly. According to CMOS, that list should be limited to: very, more, most, less, and least.
I may sometimes still feel the need to add commas to provide a visual cue to readers so they can identify a compound adjectives. I would be reluctant to have an unhyphenated compound adjective beside a single-word adjective with a comma between them.
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That all makes perfect sense to me – until it sometimes crashes onto the rocks of common sense.
One example I cannot figure out is 'high school students'. The rules suggest 'high-school' must be hyphenated to get the meaning of students who attend a high school. With a comma after 'high' the meaning is certainly students who have taken drugs but attend any type of school, but it still means that with only a space after 'high'.
Using a hyphen looks wrong to me. I think, in practice, what happens is readers interpret 'high school students' as a noun phrase. I think the only answer to that is to add the comment which exists at the end of every other "rule" of grammar, i.e. '… and if the result looks or sounds wrong, change it to something that feels more natural.' Sigh. :(