In general the closer the frequencies of two pitches the more dissonant they are. As two pitches approach each other they begin to produce beat oscillations, which are caused when the two pitches cause interference, or reinforce and cancel each others amplitudes.
Dissonance has been defined differently by different people in different places at different times. Western musical history can be seen as progessing towards a wider definition of consonance, culminating in the "emancipation of the dissonance", such as in the view of Arnold Schoenberg. Henry Cowell viewed tone clusters as the use of higher and higher overtones.
While, as can be seen from the above paragraph, the perception of dissonance is obviously culturally influenced, dissonance may also have some objective basis. Consonance between two notes can be defined as greater coincidence of their harmonics or partials, which collectively are overtones. Dissonance is then defined by the amount of beating between non-common harmonics.
In what is now called the common practice period dissonant intervals include: