By generalizing a concept on a power spectral zero at the Nyquist frequency, a condition for the line codes`` capability of signaling with the minimum bandwidth is established. In connection with the condition, a new figure of merit concerning the maximum amount of intersymbol interference in the minimum bandwidth system is introduced, and a general study is made on the minimum bandwidth systems in various aspects. By use of the new condition, three new line codes of the minimum bandwidth with different properties are designed, and their performances are compared on power spectrum, eye width, and error probability. Especially, the newly designed code called the VMDBr is shown to have most properties frequently required of the line codes, i.e., it is runlength-limited, DC-free, and of the minimum bandwidth.