What is the dual of a logical proposition?

I'm reading a paper on the geometric implications of ytical logic and it discusses the concept of the "dual" of a proposition. I would normally be inclined to think this means some sort of equivalent proposition (e.g., p AND (NOT q) = NOT[(NOT p) OR q], but it appears to refer to, say, p AND q versus p OR q. I'm familiar with the duality principle of projective geometry, but this appears to be very different. Can anybody illuminate this relationship for me? I'm not immediately seeing it.