In Western Music, we most commonly use the Equal Tempered System and the A 440 Hz standard. An octave is an interval where the frequency ratio is 2. An octave is divided into 12 semitones, with two sequential semitones differing by a fixed ratio equal to 2 raised to the 1/12 power. Two raised to the 1/12 power is an irrational number since 1/12 is irrational . Therefore, any integer power of (2 ^1/12) other than a multiple of 12 is also irrational. So A is the only note with a precisely defined value. All other notes are an approximation to whatever degree of accuracy is desired.