Take a noun, any noun, and stick two adjectives in front of it. Do you need to connect them with a hyphen? Or can you simply leave them alone?
The answer is “depends.” More accurately, it depends on what they’re doing to the noun, and what they’re doing to each other.
The Formula for Hyphenating Two Adjectives
I love algebra. And I love grammar. In many ways, grammar is the algebra of language, and algebra is the grammar of math. We can use an algebraically expressed formula for determining whether or not to use a hyphen.
We use a hyphen when the following conditions are met:
(AdjectiveA + AdjectiveB) describes Noun
AdjectiveA describes AdjectiveB
In words: We use a hyphen when two adjectives together describe a noun that follows, and when the first adjective describes the second.
Example One: oven-baked flavor
Let’s put some real words in the formula, apply them in a sentence, and see how this works.
Sentence: I love the oven-baked flavor of pretzels.
See that hyphen in oven-baked flavor? It needs to be there because our adjectives fit the formula above:
(Oven + Baked) describes Flavor
Oven describes Baked
Thus, we need a hyphen.
The hard concept to grasp is how oven and baked together describe flavor. Together, they make one meaning, one descriptive term for flavor. We can understand that they describe flavor together when we try to use them individually. If we only use one, or if we put and in between them, we change the meaning of the sentence:
oven and baked flavor is different than oven-baked flavor
oven flavor and baked flavor are both different than oven-baked.
Now that we’ve dealt with the first part of the formula, let’s look at the second part:
AdjectiveA describes AdjectiveB.
This is pretty simple, actually. What kind of baked are we talking about? The oven kind, not the old kind, the stale kind, the fire kind, or any other kind. Oven tells us what kind of baked.
We’ve satisfied the formula to determine that oven-baked flavor needs a hyphen.
Example Two: sky-blue car
Let’s briefly look at a second example: a sky-blue car.
We have a car. What color is it? Not blue but sky-blue. Here, the terms sky and blue together describe car. Using either one alone would change the meaning. A sky-blue car is different than a blue car [different color] or a sky car [different technology]. A sky and blue car doesn’t even make sense. Also, sky is describing the term blue, telling the reader what type of blue.
When we use the formula above, we get this:
(Sky + Blue) describes Car
Sky describes Blue (tells what type of blue)
Example Three: old oak tree
Here’s a case where you don’t need a hyphen: “I sat under the old oak tree.”
Here, old and oak both describe tree, but old does not describe oak—unless old oak is meant to be different than new oak. I suppose that could be possible in very specific cases, but probably not in common use. In this case, could we say “I sat under the tree that was old oak”? No? Then old is not describing oak.
For comparison with the hyphenated examples above, we could reasonably say “I drove the car that was sky blue” and “I love pretzels that have a flavor of being oven baked.”
Hey! Where’d the Hyphen Go?
When we move those two hyphens after the noun they are describing, we drop the hyphen, as in “The car was sky blue.” The sentence structure now shows how they are connected to each other, so the hyphen isn’t needed. Because the second adjective isn’t before the noun, it doesn’t give the interpretation that it can describe the noun by itself.
We also don’t need a hyphen when the first descriptor is an adverb ending in –ly. Thus, the slowly trotting horse doesn’t need a hyphen, even though it fits the formula above.
Does this really matter?
Is this important? Yes.
Of course, if your reader has fairly decent grammar and if you make a mistake with your punctuation, your reader may discredit you. But that’s not the most important reason. This is: The meaning of the sentence can change depending on whether or not you have the hyphen.
A silver looking glass is different than a silver-looking glass. The first is a looking glass (mirror) made of silver. The second is a glass that has a silvery look (appearance).
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