Slides have a unique and attractive look. When projected in a dark room, they create a certain magic. The bright colors and high contrast are what do it. I still have a slide projector, but I haven't used it in years. I have scanned many of my slides, and sometimes print them. The affect is not really the same, but you can make some great prints from them. 645 and 6X6 transparencies used to be mounted and projected as well, and projectors for these 120 sizes were commonly available. I haven't seen any, even on the used market, for years. A few projectors were made for 6X7, but that never really took off. I'm old enough to remember lantern slides (roughly 3X4 inches). With the right mask, you could project 6X9 transparencies with one of those.