The radiation from an infinite phased planar array of rectangular pistons with nonrigid interstices is considered. Numerical calculations for exact equations are compared with a simple approximation, derived for small pistons close together. The comparisons show that the approximate expressions agree well with the exact results even for pistons of practical size, if (1) the array is steered to an angle near broadside, (2) the interstitial impedance is not very low, and (3) the interstices occupy a small fraction of the total array area.