Bach's known free organ works (those not based on chorale tunes) comprise a fourth of his organ compositions but account for nearly half of their required performance time. Written more than a quarter-millennium ago, these several dozen compositions form influential and lasting standards in modern organ repertoire.

Bach's innovations in organ music were achieved mostly through applying and extending techniques used by his predecessors from Germany, Italy, and France. Bach followed Buxtehude in writing for the organ differently than for the harpsichord, yet some of his greatest contributions to the organ reflect prior developments in his own harpsichord music.

Although individual titles include Allabreve, Aria, Canzona, Concerto, Exercitium, Fantasia, Fugue, Passacaglia, Pastorale, Pièce d'Orgue, Prelude, Sonata, Toccata, and Trio, about half of the free organ works are commonly called something "and fugue." The modern notion of "prelude and fugue" is owed more directly to Johann Sebastian than most people realize. Almost single-handedly Bach established the now traditional pairing of two balanced and self-contained movements of similar dimensions, the first freer and the second more imitative. He used this structure increasingly in his organ works after exploring it first in his 48 Preludes and Fugues for harpsichord, which contain two pairings in every major and minor key of the chromatic scale. Harpsichords need frequent tuning anyway, and Bach devised a system in which a single new tuning enabled playing in all keys. His free organ works, though, represent keys less democratically. Organs--larger and more stable than harpsichords--neither needed nor permitted such frequent and radical retuning, so while Bach's harpsichord works systematically exhibit signatures from zero to seven sharps or seven flats, only a few of his free organ works use even three or four. More than half use one or none, the most frequent representation going to keys more euphoniously tuned (on organs of Bach's day at least) like C major and D minor.

The concept of two balanced movements, however, transferred perfectly to the organ. In his early works Bach often wrote short pieces or used multiple movements or sharp contrasts within movements to maintain interest, and in his later works he often used more complicated techniques to unify variety within each of two large movements yoked as a matched pair. A good example of the difference is found in the two Toccatas and Fugues in D Minor, BWV 565 and 538. The more famous BWV 565 is actually an early "Toccata con fuga"--a toccata with a fugue in the middle. The fugal section has its own beginning but not a complete ending, and each of the three sections contains contrasting textures. In the later work, BWV 538, the toccata and fugue are related but independent movements of about equal length, each exhibiting internal variety that forms an integrated whole.

Several free works constitute or contain tripartite structures. The Toccata, Adagio, and Fugue in C Major is a three-movement work with a fast-slow-fast pattern of tempos perhaps influenced by other instrumentations, as are Bach's organ trio sonatas (based in part on other works) and his organ transcriptions of string concertos by Vivaldi and others. The free-standing Pièce d'Orgue in G Major contains three sharply contrasting sections within a single movement, and the prelude of the Prelude and Fugue in D Major contains three separate sections within the first of two movements. The Pastorale in F Major, consisting of four movements, is unusual in this and several other respects.

The techniques by which Bach establishes variety in his longer fugues have pleased listeners and amazed scholars and composers for many generations. Bach sometimes assembles longer fugue subjects than do his predecessors, and then plays with the beginning, middle, and ending portions of them separately. Bach transposes the subjects to more pitch levels, uses the pedal more often as a fully independent voice, weaves more elaborate contrapuntal combinations, paces the pedal entries more effectively, and alternates subject statements with more interesting successions of contrasting episodes. After 250 years, a definitive description (however detailed or lengthy) of Bach's compositional genius seems permanently unattainable, and the fullest evidence of that genius is still to be found in experiencing the live performance of his music by a skilled interpreter on a fine instrument in a good acoustic.

Possible beginning strategies for a thoughtful modern listener include asking questions like these:

• How does this key sound on this instrument?
• How many independent movements does this piece contain?
• In the one-movement forms, is the piece clearly divided into contrasting sections?
• In the two-movements forms, does the prelude (or fantasia or toccata) balance the fugue in length and complexity, or is it shorter and introductory?
• In the long first movements, what contrasts of texture, rhythm, melody, and dynamics help make the music expressive and interesting?
• In the longer fugues, does the main subject appear on different pitches and in different variations? Are parts of the subject bantered back and forth? Is there dialog with some recognizable tune or idea that alternates with recurrent appearances of the subject?
• Do the ideas, experiments, and surprises in this piece seem more like the work of a promising young person or of a mature older one?
• Do I find some elements of variety more engaging than others at this point in my life, or do I quite simply find all the music equally beautiful?
• Which piece most motivates me to respond to the performer because of the interest, beauty, enjoyment, or meaning the music brings to my life?
• If I could e-mail the composer, which piece would I most like to thank him for?
• If I were the composer, which work would I be most proud to share with future generations?

Dr. Karl Schrock is Adjunct Assistant Professor of Organ at the Western Michigan University School of Music (www.wmich.edu), and Dean of the Southwest Michigan Chapter of the American Guild of Organists. His graduate degrees from The University of Michigan School of Music include M.Mus. in conducting (1987), M.Mus. in organ performance (1987), M.Mus. in music theory (1997) and D.M.A. in organ performance (1997).