An incident scalar plane wave is scattered from a surface, corrugated in one dimension, and given by an infinite number of periodically spaced semi‐infinite parallel plates of zero thickness (thin plates). The surface is a parallel‐plate grating. An additional plate is placed between two adjacent (in period) plates, thus dividing the periodicity interval into two regions, one of which has the same (constant) density and wavenumber properties as the material above the plates, while the second (inhomogeneous) region is filled with a material which has a different (constant) wavenumber and density from the surrounding media. The solutions of the Helmholtz equation are assumed to be upgoing plane waves above the plates and, between the plates, standing waves along the periodicity direction for either soft (Dirichlet) or hard (Neumann) boundary conditions, and downgoing waves down the slots between the plates. Continuity of pressure and velocity across the common boundary yield linear equations relating the wave amplitudes in the various regions. The equations are shown to be similar to the residue series of integrals of certain metamorphic functions. The amplitudes are expressed as values or residues of these functions, which are explicitly constructed from properties of the linear equations. Two examples are done in detail: (1) no inhomogeneity structure with arbitrary incident angle, and (2) arbitrary inhomogeneity values at normal incidence. Numerical results are presented for backscatter (specular) reflection coefficientsR0for normal incidence as a function of frequency for various values of the inhomogeneity parameters. The Rayleigh anomaly and the wavenumber threshold effects for the modes between the plates are the most prominent effects.