Originally Posted by Mediocrefunkybeat
Something to do with Gregorian Hexachords and Pythagoras I'd imagine. No I'm not joking.
Although the modern major and minor scales are strongly related to two of the church modes, the modern eight-tone scale is based on different harmonic principles and is organized differently from the scales of the church modes, which are based on six-note patterns called hexachords. The main notes in a hexachord are the dominant and the final. Depending on where the final falls in the sequence of the hexachord, the mode is characterized as either authentic or plagal. Modes with the same final share certain characteristics, and it is easy to modulate back and forth between them, hence the eight modes fall into four larger groupings based on their finals.
Just had to throw that in. Wikipedia