Counterpoint, from the Latin: “against note” is the combination of two or more separate musical lines to form an harmonic whole. It also refers to the academic discipline used throughout history to teach composers how best to combine voices in a musical texture. The term polyphony is sometimes used synonomously with counterpoint, and sometimes to distinguish medieval multi-voice music from that of the Renaissance and Baroque.
Before the 10th or 11th century, all Western music was monophonic, or consisting of only one voice, which was usually a liturgical chant. The earliest known example of polyphony, discovered in a document called the "Winchester Troper," adds voices to the chant moving at parallel intervals, such as the fifth and octave, a practice called organum. Later documents would yield examples of parallel organum at the third and sixth. The practice of singing independent lines (and often to completely different texts) began in the 14th century in France, and reached its first heights in the music of Palestrina, Byrd, Lassus and others during the 15th and 16th centuries.
The musical effect of the Counter-Reformation beginning in 1560 was to curtail the growing complexity of liturgical music, preferring homophony to polyphony. At the same time, the Lutheran church's newly-composed homophonic chorales showed an emergent sensitivity to harmonic thinking. Whereas polyphony had been primarily linear, or horizontal, in nature, this new style focused on the vertical aspect. Out of this sensibility developed the type of counterpoint which would become the hallmark of the high Baroque. New rules developed which took into account both the horizontal and vertical aspects of the musical texture, and by the time of Bach and Handel this new kind of counterpoint had been “codified” into various forms; the most notable being the fugue. This high Baroque period (generally considered to be the first half of the 18th century) became the second high point of contrapuntal writing.
Composers in the subsequent Classical period, notably Joseph Haydn and W.A. Mozart studied it, and used it extensively in the development of sonata-form movements within their works. The Romantic period saw the importance of the discipline decline, but not disappear. Mendelssohn and Brahms both found musical inspiration in its forms and disciplines. (Brahms would “doodle” with counterpoint as a cure for “writer’s block”.) The rise of Neoclassicism in many of the works of Igor Stravinsky, Paul Hindemith and other 20th century composers gave it new impetus. It is still a part of the study of composition.
Donald J. Grout and Claude Palisca, A History of Western Music
Robert Gauldin, A Practical Approach to Sixteenth-Century Counterpoint, 1985
Robert Gauldin, A Practical Approach to Eighteenth-Century Counterpoint, 1988