At top /on top

[Ahmedabbas]

How do use each?

 

[Tarheel]

How should I use each?

Well, when somebody is on the top of a mountain he's at its highest point.

 

[emsr2d2]

How do we use each "at top" and "on top"?

Note my corrections above. The forum doesn't work like this. You can't just give us random pairs of words and ask us to explain their use. You need to give us a complete sentence containing those two-word phrases and then we can help.