We study transmission properties in configurations composed of two single metal gratings with different thicknesses.Choosing the perfect electric conductor excludes the influence of intrinsic material dispersion on transmission behaviors;and as such,we aim to reveal the contribution of geometric dispersion to electromagnetic transmission.Transmission suppression line,instead of a transmission suppression point, is discovered,denoting the curve of the wavelength versus the interval or the lateral displacement between the two single gratings when the transmission suppression appears.A simplified model is proposed to comprehend the underlying physics of this special phenomenon. We study transmission properties in configurations composed of two single metal gratings with different thicknesses. Choosing the perfect electric conductor excludes the influence of intrinsic material dispersion on transmission behaviors; and as such, we aim to reveal the contribution of geometric dispersion to electromagnetic transmission. Transmission suppression line, instead of a transmission suppression point, is discovered, denoting the curve of the wavelength versus the interval or the lateral displacement between the two single gratings when the transmission suppression appears. A simplified model is proposed to comprehend the underlying physics of this special phenomenon.