Crossback

The seats are benches with crossbacks; the desks have movable tops, but these are to be replaced by stools and good stationary desks.

Source: wiktionary

This reel need not be very large, nor yet very expensive, a plain, easy-running wooden one about three inches in diameter, and costing no more than from two to three shillings, will be plenty good enough ; but one thing the tyro ought to observe, and that is, the reel should have a strong crossback to it ; the brasswork on the back should be fitted right across and in four opposite directions.

Source: wiktionary

It seems that about two hours before that the crossback in the hatch which supported the hatch covers, had fallen out, and was lying on the floor of the hold, its fall being due to an ascending draft.

Source: wiktionary

The crossback elastic strap not only gives greater excursion both in scapular abduction and in arm flexion but it helps to prevent the control attachment strap from riding over the shoulder during extreme arm flexion, such as when the amputee is working in areas over his head.

Source: wiktionary