The invention includes an adjustable media tray for an image forming device such as a printer, copier or facsimile machine. The media tray incorporates a media guide which is slidably mounted on the tray. The media tray and associated guide incorporate a releasable locking mechanism which, when engaged, permits the guide to be slidably moved in one direction, but prevents the guide from being slidably moved in the opposite direction. The locking mechanism incorporates a plurality of pawls and at least one ratchet rack with equally-spaced, ramp-shaped teeth. Each of the teeth has a vertical edge. Each of the pawls has associated therewith a plurality of teeth of a single rack, and each of the pawls is adapted to sequentially engage a vertical edge of each of its associated plurality of teeth. Engagement of each pawl with the vertical edge of each of its associated plurality of teeth is out of phase with engagement of each other pawl with the vertical edge of each of its associated plurality of teeth. The out-of-phase arrangement is provided either with multiple, offset pawls which operate on the evenly-spaced teeth of a single linear ratchet rack, or with multiple offset racks, each of which has associated therewith a pawl. In either case, the amount of offset between the pawls of the former arrangement or the racks of the latter arrangement is equal to the period, or distance, between teeth, divided by the number of racks or pawls.