Middle C in a 4' stop sounds one octave higher and a 2' stop sounds 2 octaves higher. All of the whole numbered stops sound at octaves which means that if you play middle C using a 16', 8' and 4, you will only hear the note C, but at three different pitches levels. Most of the stops on the organ sound at octaves with the fundamental note. This is crucial because the music would be very dissonant if playing the middle C key would sound a C and a B. Stops labeled with a mixed number or a Roman numerals do not sound in octaves but are not dissonant with the fundamental note. However, it is not the same note name as the fundamental so playing middle C does not yield the note C at a different pitch level.
Mutations give the organ a rich, bright sound.
Stops with 3 as the denominator of the fraction always produce a fifth above the fundamental note. This applies to any note. The seventh note from the starting point is the fifth of that note. To figure out the third, count up 4 keys from the starting key. The mutations do not actually sound a simple fifth or third above the fundamental but usually sound above the 4' pitch level.