All of the travel, study and exposure to other musicians and music gave him an insight into the craft that made him a great performer, composer and teacher. Indeed, his duties at St. Thomas Church in Leipzig, a position that he held from 1723 until his death, required that he supervise and provide the music for 4 churches in the community, play the organ as well as teach the choir and instrumentalists. Bach must have had a robust constitution most of his life, for he was a very busy man.
He taught his herd of children as well. some of them went on to make music their life and were very influential in their time. Johann Nikolaus Forkel wrote the first biography of Bach in 1802. Forkel knew some of his children, and C.P.E. Bach especially gave Forkel insight into the elder Bach's teaching methods. Forkel wrote:
To teach well a man needs to have a full mind. He must have discovered how to meet and have overcome the obstacles in his own path before he can be successful in teaching others how to avoid them. Bach united both qualities. Hence, as a teacher he was the most instructive, clear, and definite that has ever been. In every branch of his art he produced a band of pupils who followed in his footsteps, without, however, equaling his achievement. For months together he made them practice nothing but simple exercises for the fingers of both hands, at the same time emphasizing the need for clearness and distinctness. He kept them at these exercises for from six to twelve months, unless he found his pupils losing heart, in which case he so far met them as to write short studies which incorporated a particular exercise.The Well-Tempered Clavier was written with this in mind, as well as being an example of how a well-tempered tuning of keyboard instruments opened up the possibility of playing in all 24 major and minor keys. As Bach wrote in the preface to the work:
...for the profit and use of musical youth desirous of learning, and especially for the pastime of those already skilled in this study.Prelude and Fugue No. 13 in F-sharp Major, BWV 882 - A prelude of persistent dotted eighth note patterns. It is in 2 voices with the dotted note patterns appearing in almost every bar, usually in one voice or the other, seldom at the same time. The mood is maintained throughout until a few bars before the end when the music drifts into F-sharp minor. But this is momentary, and the prelude ends with a cadence to F-sharp major.
The subjects of Bach's fugues are like the seeds of plants. Some are simple, some are not, most of the subjects of his keyboard fugues are short, mainly because they were meant to be played on the non-sustaining keyboards of the harpsichord and clavichord. The fugues written for organ for the most part have longer subjects. The subject of this fugue is unusual in that it begins on the leading tone, that is the seventh note of the F-sharp major scale, instead of a note within the F-sharp major triad. This doesn't guide the ear to the key of the piece, but rather away from it. The music winds its way, but Bach brings it all to a satisfying conclusion.
Prelude and Fugue No. 14 in F-sharp Minor, BWV 883 - This prelude is in 3 voices, but they are not equal in importance. The upper voice is dominant, and the music floats at an easy pace, with tinges of melancholy here and there. A little over halfway through, Bach stops the music altogether for a pause that leaves the music hang in midair. The music returns and makes its way to the final cadence in F-sharp major.
Bach's fugues are challenging not only for the technical aspects of playing the notes. There is also the questions the performer has to make on which is more important in each fugue; the harmonic structure, melodic structure, phrasing and so on. For a performance to be all it can be of any of these works, a performer needs to make decisions. That is why there are so many books written about the WTC, there is a wealth of ideas contained in each pairing of prelude and fugue, and a tremendous amount in the entire two book set. The subject of this fugue is a little longer than three bars, and it is is in 3 voices. There is much melodic content in the fugue besides the subject itself, and Bach uses the subject as the thread to tie it all together.The running sixteenth notes continue in this novel subject of 5 measures. This subject statement is heard only 6 times throughout, with the balance of the music taken up with counter subjects and episodes. In its liveliness, it is a perfect accompaniment to the prelude heard before it.
Prelude and Fugue No. 16 in G Minor, BWV 885 - This prelude is one of three in the entire set of 48 to have a tempo designation, Largo. Bach was adamant that this was not to be taken fast, but slow and stately.
Beginning on the second beat, the subject of this 4-voiced fugue has the 4th degree of the G minor scale (C) played out seven times at the end of it. As soon as the seventh C is sounded, the counter subject begins before the subject makes its second entry. The entry of the other two voices follows this pattern. The subject matter returns many times with changes in key, sometimes it plays against another version of the subject heard in a different voice. The music does end with a Picardy third (that is, with a major chord) but just barely. The final note of the fugue is a major third, B natural.
Prelude and Fugue No. 17 in A-flat Major, BWV 886 - A prelude of calm and sweetness of harmony. Although it is for the most part in two parts,s there are instances of chordal structures within it.
The subject of this 4-voiced fugue begins after an eighth rest and lasts for two measures. The seeming rhythmic simplicity hides the subtle syncopations within the piece.
Prelude and Fugue No. 18 in G-sharp Minor, BWV 887 - This prelude has unusual dynamic designations in it not normally found in the The Well-Tempered Clavier. In the third bar there is the term piano, while in the 5th measure there is marked forte. Perhaps this was meant for a harpsichord that could allow for different dynamics, or on the clavichord which was capable of slight variations in dynamics. The rest of the prelude is not so marked, but perhaps Bach did it to make the point that there were to be echo effects to be played in this prelude whenever the music demanded it and the instrument allowed it. There are two sections, almost of equal length; the second section has two more bars than the first. The prelude is mostly melody and accompaniment with little counterpoint involved.
The subject is 4 measures long, entirely in 8th notes, and the fugue looks remarkably plain on the printed page, but roughly half way through the fugue, Bach introduces a second subject that is more chromatic and of a different rhythm than the first. This helps the listener detect changes between the first half and the second half of the fugue, and helps avoid monotony. Each subject enters and leaves with differing voices, and aided by syncopation, they add variety. There are no increases of tension, no contrasts of major and minor. By the use of chromaticism and different motifs for each subject, as the subjects themselves, to create a mood of subtle color shifts.