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We report on the transmission of terahertz (THz) electromagnetic waves through gold grids evaporated over a transparent silicon substrate. We observe a near-unity transmission in the subwavelength regime. When the THz wavelength is comparable to the grid periodicity, the grid serves as a grating coupler that allows us to excite guided modes in the device. These guided modes exhibit an intermediate behavior between a surface plasmon mode and modes of a dielectric slab waveguide.