What is the difference between on the decline and in decline As I observe, the preposition on follows by definite article while the other does not.

Are both use replaceable?

  • 1
    You should add some actual usage sentences where you found these terms. It might help you evaluate potential differences.
    – user3169
    Aug 6 at 2:23

Both phrases are commonly used, as well as, "on a decline". AFAIK, they have identical denotation and connotation, and can be used interchangeably... As well as "He's going downhill."

Oddly, "It's going downhill," can mean, "It's not doing as well as before," or, paradoxically, the antithesis, "It's getting easier," as in walking downhill after a long trek to the summit.

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