Musical sound is the result of regular and periodic vibrations of air.

The varying height or depth of sound is caused by the difference of the rapidity of these vibrations, and is called the pitch of the sound.

The loudness or softness of a sound depends upon the size, or amplitude, of the vibrations. This is called the force of the sound.

The quality or ‘timbre’ of a sound depends upon the nature of the vibrating body (i.e., the "medium by which vibration is set up, whether string or column of air) ; and also upon the presence, in greater or less degree, of harmonics.

The indication, in writing, of musical sounds requires:
  1. Notes, to express duration.
  2. The staff or staves
  3. Clefs

The latter two express pitch.