AZO is a silver chloride paper whereas most papers are silver bromide based. Silver chloride is sensitive over a braoader spectrum so the negatives UV characteristics are a factor. AZO is generally not an enlarging paper so it is moslty used with big negs - 4x5, 5x7, 8x10 etc. AZO can record a greater span of tones and has a very straight response to light - very little toe and very little shoulder. And when developed with Amidol, it has a very very dense cold black. Contact prints on AZO have increadable tonality and depth - they look 3d. I bought some AZO and Amidol and contact printed some 4x5 negs and within a month I was setting up for 8x10! It was AZO that led me there. It is an amazing paper ... and quite collectable too - Prints made on AZO often sell for more than enlargements. Collectors know the difference.