Behind the side-by-side cylinders of equal diameter, the basic flow structures are categorized as follows. First, only one single vortex street exists if they AT 1015 are spaced very closely (G∗ < 0.3). In this case, the shedding frequency is about half of that behind a single cylinder at the same Reynolds number. Secondly, a vortex street behind each cylinder is observed while the gap ratio (G∗) is equal to or greater than 2.5. These two vortex streets interact very weakly and may shed in phase or out of phase at random time intervals ,  and . Third, while two cylinders are spaced with intermediate gap ratios, the flow structures depend upon the Reynolds numbers, the gap ratio and the experimental conditions . For the gap ratio 0.3 < G∗ < 2.0, formation of a wide and a narrow wakes behind each cylinder is observed  and . The well known flow pattern is the stably biased gap flow. Under some situations, the gap flow may switch upwards or downwards at intermittent and random time intervals , ,  and . The switching timescale is several orders of magnitude longer than those of the vortex shedding and the shear layer instability . From the practical viewpoints, the third category receives more attention because the loadings (mean, fluctuating drag and lift) on the cylinders upstream of the narrow and the wide wakes exhibit appreciable differences.