The tuning system used in the West today is called Twelve-Tone Equal Temperament.  One arrives at this tuning by simply dividing the octave into twelve equidistant semitones.

Modern tuning terminology uses the word “cent” to describe 1/1200th of an octave.  If an octave is divided into twelve equal semitones, then each semitone is positioned at a distance of 100 cents from the previous one.  Thus, a tuning table for Twelve-Tone Equal Temperament looks like this:

SEMITONE	SIZE	POSITION	INTERVAL NAME
1 (C)	100	0	Unison
2 (C# or Db)	100	100	Minor second
3 (D)	100	200	Major second
4 (D# or Eb)	100	300	Minor third
5 (E or Fb)	100	400	Major third
6 (F or E#)	100	500	Fourth
7 (F# or Gb)	100	600	Augmented fourth/Diminished fifth
8 (G)	100	700	Fifth
9 (G# or Ab)	100	800	Minor sixth
10 (A)	100	900	Major sixth
11 (A# or Bb)	100	1000	Minor seventh
12 (B or Cb)	100	1100	Major seventh
13 (C)	100	1200	Octave

It is evident that each succeeding semitone is the same distance from the preceding one.  If one was to play all the semitones in a row, he or she would be playing a Chromatic Scale.  Here is an example of it, played on a piano through two octaves. 

To find out how Western music arrived at Twelve-Tone Equal Temperament, click here to go on to the next page.