A vibrating String.
A resonating column of air.
A vibrating membrane.

With the advent of Electronic Music and the technological advances in music reproduction and recording a further way of producing a music pitch has become extant:-

An oscillating electrical signal.

Musicians measure Pitch generally with letter names -- A, B, C, etc. and further refine these divisions with things called Sharps and Flats. In western music this is further complicated by a series of Major and Minor Keys, Intervals, Chords and Triads.
In order to fully understand the concept of Pitch and the important role it plays within a musical composition, it is necessary to study what actually a pitch is. A Pitch is produced when something makes air vibrate regularly at a particular frequency. If the air vibrates at less than twenty times a second or greater than twenty thousand times a second humans cannot hear the sound as the human ear cannot process sounds outside that range. With that in mind it can be seen that the human has a very limited hearing range of 20 to 20,000 vibrations per second. Stated thusly this is called "Frequency" (not pitch) and is measured by the acoustic science community in Hertz (Hz.) (after Heinrich Rudolph Hertz (1857 - 1894) who invented the measurement system) or "Cycles Per Second" (cps).
In order to make the transition from frequency to pitch another natural scientific (or perhaps Psychacoustic) phenomenon needs to be stated. The Human perceives a frequency of 880Hz as the same frequency as 440Hz but higher. This is true for any frequency and its double. In music this relationship is called an "Octave" and it is this relationship that is the basis of western music to this day. Initially the "Octave" was divided into eight discreet divisions each representing a note. This accounts for the name "Octave". The series of "notes" created by dividing the "Octave" into eight were called "Modes" and were prevalent until about 1600. Each "note" created by this division was given a name representing a letter of the alphabet -- and thus the Musical Alphabet was created as seen below:-

A, B, C, D, E, F, G, A.

During the Seventeenth and Eighteenth Centuries composers became more and more disenchanted with the "modal system" of pitch organization as it was very restrictive and did not allow for pitch or key innovation. By 1750 an additional division of the "Octave" was introduced represented, at the time, primarily by the music of J.S. Bach (1685 -- 1750). This system further divided the "Octave" into twelve equal parts and thus the "Chromatic Scale" was born. The tuning of this system of equal division of the "Octave" was called "equal temperament" and continues to this day. It provides for the composer the flexibility to move from key to key (modulate) and to enrich the harmony palate of any composition.It should be emphasised, however, that primarily the equal temperament system was a system of tuning keyboard instruments and not a theoretical pitch system in itself.
Since the introduction of the Chromatic Scale a number different series of pitch organizations and refinements of existing pitch organizations have been introduced. Broadly they fall into the following catagories:-

Modal system of Pitch organization.
Tonal system of Pitch organization.
Atonal system of Pitch organization.
Micro-tonal system of Pitch orgainization.

As indicated above these are very general and broad titles and within them there are many different variations of pitch organizations. For more information on each broad title click on the link above.
Within the compositions of Kerry R. Scott there are examples of all the systems of pitch organisations listed above. Follow the links below for information, recording samples and scores on the various CD's that contain these systems of Pitch Organization.

Electronic Music

Music Notation

Temporal Notation

Equal Temperament

Rhythm

Timbre

Dynamics

Sound Sculpture No. 1 [atonal pitch systems]

Trio for Bass Clarinet, Trumpet and Percussion By Kerry R. Scott. [atonal pitch systems]

Follow the following links for more information about the topic and navigate the:-
ScottOpus Productions Website

Further information on Kerry R. Scott's life and Compositions
A listing/portfolio of the music compositions of Kerry R. Scott
Further Information on The Tudor Rose School of Music
Further information on the Music CD -- Bubble and Squeak [atonal,tonal pitch systems]
Further information on the Music CD -- Rattle and Rhyme [atonal,tonal pitch systems]
Further information on the Music CD -- Beyond the Virtual Creation [atonal,tonal pitch systems]
Further information on the Music CD -- Brandy Butter, Brass and Bells [Modal, atonal and tonal pitch systems]
Further information on the Music CD -- The Old, The New, and an Eclectic Medley [Modal, atonal and tonal pitch systems]
Further information on the Music CD -- Smphony No. ! -- Soundscapes of a Forgotten Britian [modal, atonal and tonal pitch systems]
Further information on compositional and composition commissions.
Further information on the stories and writings of Kerry R. Scott.

Web scottopus.org