A frame of rectangular configuration, constructed of structural components and usable in connection with modular furniture. Front and back frame components are interconnected by a pair of side components, and a locking bar member extends between the side components and is movable therealong while remaining essentially parallel to the back component. The locking bar member has a straight edge in alignment with and parallel to a straight edge of the back component, such that when the locking bar member is brought into a relatively close relation to the back component, the straight edges of the back component and the locking bar member can hold therebetween a backrest equipped with cleats in a secure, fixed position, without the need for screws, bolts, pins or other joinder elements.