A tray keying mechanism is mounted on a backplane inside a modular chassis and has a protrusion that stands out like a post for each of several equipment bays. A modular tray can be inserted into each of the equipment bays, but it will not be allowed to be fully inserted and to engage the electrical connectors on the backplane if the corresponding post meets an obstruction on the rear apron of the tray. If the particular tray is the correct type, and is not upside down, the post will encounter no obstructions during the insertion of the tray, and the electrical connectors will be allowed to engage. In one embodiment, the keying mechanism comprises a bracket of molded plastic that fits all around the several connectors on the backplane, and could be patterned to fit an already existing and in-service backplane and chassis.