Answers (1)
Music is a mess. If you sing evenly spaced notes, there are eight to an octave. Somebody else might have a higher voice so they sing eight different notes. You wonder why they don't sing the same notes but starting on a higher one? Well, it turns out that an octave is divided into twelve notes, and even that is a simplification.
So out of those twelve notes you find a pattern: people all start on or pretty close to one of the notes and then sing two half steps and a whole step, three half steps and a whole step. Variations on that pattern are called "church modes". They don't have to have anything to do with church, except that what we call western music was invented by Christians to glorify their God, and they investigated every possible variation on everything, especially church music.
Examples: www.youtube.com/watch?v=pZpeYO3iat8