An experimental study was performed to investigate the effects of the spacing ratio between the nozzles on the evolution and interaction of twin rectangular jets. The velocity measurements were conducted using particle image velocimetry (PIV) technique. The measurements were performed on the symmetry plane of the rectangular jets. A total of 5 test cases of S/de = 1.8, 2.8, 3.7, 5.5 and 7.3 were examined with Reynolds number being fixed and equal to Rede = 10,000. The effects of the spacing ratio on the mean flow in terms of its mixing characteristics (e.g., decay and spread rates) and turbulence statistics (e.g., turbulence intensities and Reynolds stresses) were investigated. The focus was then on large-scale structures (LSS). The evolution of LSS was studied along the shear layers and the symmetry line in order to investigate the effects of nozzle spacing and interactions. For doing so, various techniques such as Galilean decomposition, two-point correlation and swirling strength were applied on the flow field. The conditionally-averaged mean streamwise velocity profiles were also extracted around the turbulent/non-turbulent interface (TNTI) and results were compared with twin round jet counterpart. The results showed that changing the spacing ratio directly affects some characteristics of the mean flow, turbulence statistics and LSS. For instance, it was observed that increasing the spacing ratio, linearly increases the streamwise locations of the merging and combined points though with different slopes. Turbulence intensity profiles along the symmetry line showed a peak downstream of the merging point, where its value decreased as the spacing ratio increased. Also, interestingly for S/de ≤ 3.7, the peak of the transverse component was larger than the streamwise component. Regarding the integral length scale of turbulence along the symmetry line, a linearly increasing trend was observed. Whilst no effects of nozzle spacing was observed on the streamwise component, the trend for the transverse component showed to increase by increasing the spacing ratio. Due to the importance of the shear layers in free shear flows, a special attention was paid to the inner and outer shear layers. In the converging region, jet half-velocity widths were identical for the shear layers. In the merging region however, while the outer shear layer continued to spread similarly to its single jet counterpart, the half-velocity width of the inner shear layer rapidly increased before becoming unstable by the interactions. Study of coherent structures showed that the effects of interactions become more pronounced as the inner shear layer develops in the merging region. For instance, 0.1de downstream of the merging point, the streamwise spatial correlation of the outer and inner shear layers were identical. By progressing into the merging region however, this reduced for the inner shear layer while expectedly increased for the outer shear layer. In a similar fashion, at the TNTI the difference between conditionally- and unconditional- averaged profiles of mean streamwise velocity increased as the inner shear layer developed in the merging region. As a result, the conditional profile jumped to a larger value upon entering the turbulent region.