Greek Music

"The four sides are fashioned for the same reason that the tetrachord is believed to be attributed to Mercury. Indeed, this number signifies either the four regions of the world or the four seasons by which the year is encompassed, or the fact that the zodiac’s structure is defined by two equinoxes and two solstices, just as Apollo’s lyre with its seven strings represents the movements of the seven celestial spheres, over which nature has established the sun as ruler."