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A theoretical and experimental study is performed to investigate unsteady, two‐dimensional, incompressible fluid flow over both sides of a slot‐perforated flat surface, which is placed in a two‐dimensional channel. The governing boundary‐layer equations are discretized by means of a finite‐difference technique to determine streamwise and transverse velocity components. The roles of both the Reynolds number and the ratio of the slot width, d, to the plate thickness, δ, on the velocity field are disclosed. It is found from the study that: (i) the flow pattern between two plates can be classified into four categories depending on a combination of Re and d/δ, (ii) at a small value of Re and/or d/δ, flow over the slot exhibits no timewise variation, (iii) when Re and d/δ exceed certain values, an alternate crossing of flow from one side of the plate to the other occurs across the slot, and (iv) a further increase in Re results in a complex flow both inside the slot and on the plate downstream of the slot. These results are confirmed by the flow visualization using ion‐exchange resins.