Some shorter prepositions sometimes function as adverbs. In such cases, they should be capitalized like any other adverb in titles, headlines and such.
How can you tell if it’s a function or preposition? Look for the noun. If the word is in a “relationship” with a noun, it’s a preposition and should stay “down” (lowercase).
“Sittin’ on the Dock of the Bay”
Jones to Update Attendees on Legislative Matters
If it’s modifying a verb, it’s an adverb. Uppercase it.
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