A chassis for telecommunication gear including horizontal circuit boards includes an enclosure forming a front-accessible circuit board slot for receiving horizontal circuit boards. The circuit board slot has first connectors for receiving mating second connectors on the circuit boards, and multiple circuit board guides for guiding and securing horizontal circuit boards inserted into the slot. Adjacent pairs of the guides are spaced to receive circuit boards of predetermined widths, and at least one of the guides is removable so that it can be re-positioned within the slot for changing the spacing between adjacent pairs of the guides to receive circuit boards of different widths. The circuit boards may be telecommunication line cards.