Full Form of CYN

What is CYN Full Form?

CYN is full form Can You Not?

What is Can You Not??

Can't you tell just by looking? [I read this from a comic-detective series]
Can you not hear me? [I heard this from an animation movie]
So, does it make a difference when you use can't you and can you not? Or anything else like it, such as is it not and isn't it?

I also find that question tags often use those two types of tags, which in my mind are somewhat not interchangeable, such as:

It's surprisingly hard to find, is it not?
It's surprisingly hard to find, isn't it?