There are several types of tray (also called magazine or holders) that are used by slide projectors. The common one is the Universal (U) - a straight magazine with a capacity of 36 or 50. This is the type to use if you have a collection of slides in mounts of varying thickness. Then there is the CS system developed by Agfa for their special CS mounts. These are very thin mounts with grooves that slide and lock securely into the tray so they don't fall out when the tray is tipped upside down. 100 can be held in the same space as a 50 capacity universal tray but the mounts have to be CS type. A tray offering a best of both worlds approach is LKM (designed by Leitz Kindermann) these hold 80 of any make of slide mounts, providing they are 1.6mm or thinner, and take up the same space as a universal 50 capacity tray.