A fiber optic cassette system is disclosed comprising a case defining a tray receiving space and an opening to a space, a tray received within the case for movement between a stored position wherein the tray is inside the case and an accessible position wherein a front end of the tray is in front of the opening and outside of the case, fiber optic cassettes arrangeable side by side along the tray front end, a first of the cassettes comprising a width of one of one, two, three, four or six standard width units and a second of the cassettes comprising a width different from the first cassette width and one of one, two, three, four or six standard width units, and a fastener for removeably the cassettes to the tray. When arranged on the tray each of said selected cassettes is directly adjacent at least one other selected cassette.