Transmission line data for the even-mode characteristics of a bilateral slotline (double-sided slotline) are presented. The slotline is analyzed by the spectral Galerkin method, and results are presented for the slot wavelength and characteristic impedance for substrates having εr = 2.22, 3.5, 6.0, 9.8, and 12.8. The results are presented for 0.01 ≤ √εr - 1d/λ0 ≤ 0.25, where 2d is the substrate thickness and εr its dielectric constant.