Question about the sign of output voltage in clipper circuits

[iampaul]

I have attached an image of a clipper circuit below.

First Question: When the sinusoidal input is positive or negative, how do i know how to label it in the

 

[Jony130]

This picture tells all you need to know about AC current

Any more questions?

 

[iampaul]

sorry for the post, it isn't complete, i was interrupted by my classmates while typing.
Here's the continuation:
What I'm actually trying to ask is how do i label the positive and negative signs. I can't express myself clearly in English so i'll just give the situation: My professor labels the upper terminal in the input side as + and the lower terminal as - when the input voltage is positive and when the input is negative, the positive and negative signs are reversed. how do i know how to label the terminals when the input is positive or negative. I'm confused because, if i flip over the circuit, doing the opposite is also valid...

 

[iampaul]

Just to clarify my question: Vin in the image I've given is positive and my professor has labelled the upper terminal in the input as + and the lower labelled as -. When the input is negative the + sign goes down and the - sign goes up.
Is it not possible that the when Vin is positive, the upper terminal is - and the lower is +.
If i flip the circuit backwards, this is the result.

 

[Carl Pugh]

Assume that the input is DC instead of AC.
If the input is +20V what is the output?
If the input is -20V what is the output?