D.C. Mun. Regs. tit. 24, r. 24-3209
In buildings with windows where the distance between anchors is greater than six feet (6 ft.), special approved anchors shall be installed at each side of the window frame. These anchors shall be set forty-two inches (42 in.) above the window sills.
When extra-width windows are being cleaned, a brass cable or chain that can readily be attached to the anchors to be used as a back support for the window cleaners shall be provided and used.
Standard belt anchors to which the window cleaner shall attach one (1) end of his or her belt shall be installed above the special anchors on extra-width windows.
Where masonry openings for mullion windows are more than five feet six inches (5 ft. 6 in.) wide, at least one (1) anchor fitting shall be installed in each mullion.
Casement windows that cannot be wholly cleaned while standing at the inside shall have anchor fittings installed in the same manner as specified for other windows.
Provision shall be made so that the outside of horizontally projected or pivoted windows may be cleaned without necessitating the window cleaner's leaning against, or putting his or her weight on, the sash.
D.C. Mun. Regs. tit. 24, r. 24-3209