Bach - The Well-Tempered Clavier, Book I, Nos. 13 - 18

There is much discussion in classical music circles whether to play the Well Tempered Clavier on the modern piano or on the instruments of Bach's time. There is no evidence that Bach had any particular keyboard instrument in mind when he wrote the WTC. Harpsichord, clavichord, organ, even a little-known keyboard instrument called the lautenwerk (lute harpsichord) that had gut strings and sounded like a lute, all could have been used to play the pieces.

The piano was still in its infancy in Bach's time, but he did play the improved pianos of the organ builder Silbermann and liked them. The important thing to remember is that whichever instrument is used, it is the music that needs to be brought to life by the musical taste, intelligence, and technique of the performer. 

Prelude and Fugue No. 13 In F-sharp Major BWV 858 - 
This prelude is in the key of F-sharp major, one of the most complex key signatures that was made available for keyboardists with the tempered keyboard tunings in vogue. It is short, and written in the uncommon time signature of 12/16 to facilitate the ease of reading and to remove the necessity of including triplet notation.is similar in style to Bach's two-part inventions:

The subject of the corresponding fugue for 3 voices is two bars long. This subject is heard eight times throughout the fugue, and there are two counter subjects. 

Prelude and Fugue No. 14 In F-sharp Minor BWV 859 - 
Some of the preludes of the WTC are fugues in their own right. Such is the case with this one. It is a strictly written fugue for 2 voices.

The subject of this fugue for 4 voices is four bars long. The mood seems to be one of calmness.

Prelude and Fugue No. 15 In G Major BWV 860 - 
The 24/16 time signature and arpeggiated chords in triplets gives no doubt that this prelude is to be played at a lively tempo.

The fugue is for 3 voices and is a perfect partner to the virtuosic prelude. The subject of the fugue is 4 bars long and is heard many times during the course of the work. Some of these recurrences are incomplete repetitions, and there are numerous episodes. All of this makes for one of the longest and most complex of all the fugues of the WTC.

Prelude and Fugue No. 16 In G Minor BWV 861 - 
The overall calm nature of this prelude is given spice by the opening trill in the right hand.

The 4-voiced fugue that follows is slow in tempo and tension builds up by the many repeats of the subject. This tension is relaxed at the end by the ubiquitous Picardy third.

Prelude and Fugue No. 17 In A-flat Major BWV 862 - 
The opening motive heard in the right hand dominates the prelude and is heard in different harmonic guises almost throughout. An example of how Bach could write music by using the most elemental and short musical motives.

The subject of the 4-voiced fugue is short, and like the motive of the prelude is heard numerous times throughout.

Prelude and Fugue No. 18 In G-sharp Minor BWV 863 - 
Another key that was once thought of as theoretical on account of its many sharps, it is an enharmonic equivalent for A-flat minor, a key with 7 flats. It is essentially in 3 parts, with a feeling that it should not be played too slowly.

The fugue is for 4 voices, with a subject that is two bars long. Outside of harmonic changes, this subject is heard with none of the usual alterations heard in many fugues. It is a fugue that doesn't seem to be going anywhere, and there is little tension created because of the verbatim repetitions of the subject. Nonetheless, the appeal of the subject maintains interest.