A simple model for the intersymbol interference due to a low-frequency cutoff channel is presented. The model is used to develop a computationally efficient and accurate method for computing symbol error probability for alphabetically line-encoded sequences transmitted over this channel. The model is also used to design alphabetic line codes that minimize the symbol error probability. A method is presented for comparing codes of different efficiencies (and, hence, for a given information rate, different transmission bandwidths). Sample calculations are then presented for two commercial high-capacity baseband digital transmission systems.