The voltage between the two lines of a single-phase electrical power system or between any two lines of a symmetrical three-phase system.
In the case of a symmetrical six-phase power system the six lines can be considered as arranged around the periphery of a regular hexagon in correct order of phase sequence. Then the voltage between any two consecutive lines is the hexagon voltage. The delta voltage is the voltage between alternate lines and the diametrical voltage is the voltage between lines arranged opposite to each other on the hexagon.