This paper attempts to take a fresh look at the behaviour of simple reinforced concrete beam–slab systems, subject to gravity loading. The simplest case of a square slab, integrally connected to edge beams and supported on pillars or columns at the four corners, is considered. It is shown that the usual design procedure of separating the slab analysis and design, from that of the edge beams (proportioned to be adequately stiff), is irrational in terms of expected behaviour at collapse. The expected diagonal yield line formation in the slab is kinematically incompatible with the expected plastic hinge formation in the edge beams. This paper attempts to resolve this dispute in design by showing how the mode of failure depends on a relative beam–slab strength parameter. The yield line theory, which considers the alternative possibility of combined beam–slab failure, is validated by experimental results reported in the literature. It is established that the prevailing design practice, assuming diagonal yield line formation in the slab, turns out to be not only irrational but also uneconomical. The combined beam–slab failure mechanism is more likely to occur in practice, and it would be rational and economical to aim for such a design.