Long, Hot Summer
When each of two adjectives modifies the same noun, put a comma between them. As a test, put the word and between the two adjectives. If it fits, then the adjectives are called coordinate adjectives:
It is going to be a long, hot summer.
She was a faithful, sincere friend.
Here's the exception: If the first adjective modifies the idea expressed by the combination of the second adjective and the noun, do not use a comma between the two adjectives:
He favored traditional political institutions.
In this example, the expression political institutions is a single idea. Thus, the two adjectives (traditional and political) do not equally modify the single noun. Rather, the first adjective traditional modifies the single unit political institutions.
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