This page gives a very quick whistle-stop tour around how the western system of music came to be the way it is through rationale, this will likely miss out some information that some might think to be useful and I'm not taking a historic tour (I won't be talking about Greek tribes here at all)
Sound is a Vibration
As you may well already know, sound is a vibration that propagates through the air and is picked up by our ears. A vibration is easy to quantify in a solid - the solid object oscillates rapidly, physaically moving in small distances back and forth
Sin Wave
Natural Sounds
Overtones
Equal Temperament
Major Scale