Example:
He was standing on the shoreline.
He was standing at the shoreline.
He was standing by the shoreline.
What's the difference between the three?
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Not very much difference at all – not with three mundance sentences like those. Without any further context, I'd say they are pretty much interchangeable. If you want to get into any subtle nuances or differences between these three prepositions, you'd have to add a lot more detail; even then, they might still be largely interchangeable. – J.R.♦ Jan 16 '15 at 10:01
If we use this definition of shoreline, then:
He was standing on the shoreline.
This is OK because we are standing on top of a surface, though on could have other meanings such as along.
He was standing at the shoreline.
Means the same thing but it refers to location (where the water meets the land).
He was standing by the shoreline.
Somewhere nearby or close to the shoreline.
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-1 for on is "automatically correct because we are standing on top of a surface." Sure, "on the shoreline" is correct, but we miss one very important point here: on has more meanings than "on top of" – many more meanings, in fact. In this case, "standing on the shoreline" is correct, not because he is on top of the shoreline, but because he is standing close to or alongside of the shoreline; see Meaning #4 at Collins. If on means "on top of," then what does "a house on the ocean" mean? – J.R.♦ Jan 16 '15 at 10:07
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