A compound modifier requires a hyphen between two or more words modifying a noun.
What are the following constructions called?
He suffered first- and second-degree burns.
5- and 6-year-old students showed up today for class.
In other words, this sentence suspends the use of degree.
He suffered first-degree and second-degree burns.
And this sentence suspends the use of year and old.
5- and 6-year-old students . . .
Notice with suspensive hyphenation:
There are only two hyphens
There is a space between the first hyphen and the “and”
There is only a space after the and
There are no spaces on either side of the second hyphenHe suffered first- and second-degree burns.