Flow around two square cylinders is investigated by the measurement of the surface pressure, the flow visualization and the supplementary numerical simulation. Two different types of flow patterns are observed which are characteristic of staggered arrangements. In either case, the gap flow is formed between two cylinders, which is biased towards the upstream cylinder. The biased flow then undergoes two different changes in the subsequent period depending on the relative location of cylinders. In the first case it flows on straightforward and finally builds up a strong vortex behind the upstream cylinder. In the second case it changes the direction and deviates towards the downstream cylinder. A strong vortex is now built up behind the downstream cylinder. However, the second flow is not stable and can not exist by itself. The fluctuation between the first and the second flows always appears in this case. The transition between the two flow regimes is abrupt under the continuous change in the cylinder arrangement.