A compound modifier consists of two words connected by a hyphen, which act together like one adjective. Usually, compound modifier words could be understood as individual modifiers or nouns, so the hyphen is required to clarify the function of the words.
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Consider the sentence below:
It’s recommended you not take down any load bearing
walls when renovating.
In the sentence above, it sounds like you shouldn’t take down any load that is holding up a wall. A hyphen should be inserted between load and bearing, so the reader understands not to take down any walls that are holding up a load:
It’s recommended you not take down any load-bearing
walls when renovating.
Look at the following examples of sentences featuring compound modifiers connected by hyphens:
This rock-hard
cake is absolutely impossible to eat.
The quick-witted
boy had everyone on the bus laughing.
No one likes a two-faced
person.