I believe most of pianists know how to play at least a part of the Well-Tempered Clavier. However, some of them may not know the reason why Bach composed it. The reason behind it is interesting.
In the old time, there are two systems of musical tuning – meantone temperament and equal temperament. In meantone temperament system, each perfect fifth interval is smaller than what we hear on piano in these days. If I compose a piece in C major, it will sound slightly different in G major. However, the piece will probably sound quite different in F sharp major there are six fifth intervals between C major and F sharp major.
In equal temperament system, there are twelve semi-tones in one octave and each of them is equal. It is exactly what we have in these days. Bach’s Well-Tempered Clavier covers all keys. His intention was to show people that we can play the music in any keys and it will sound agreeable.
There are a number of reasons why I think Beethoven is the greatest composer ever lived. The first reason is that he was able to summarise the music ideas that existed in his time and paved the path for composers in romantic period. The fourth movement of his piano sonata No. 29 in B flat major demonstrates the use of Fuga, an older style of music form, with a new sense of freedom. In other words, instead of abolishing the music forms in Baroque period, he was able to summarise and bring it to the future.
Also, there is something special about Beethoven’s adaptability. It is not difficult to notice that he was capable for using difference motifs to expand music ideas in his piano sonatas. For example, the third movement of his piano sonata No. 14 in C sharp minor is an excellent demonstration of arpeggios but repetitive and static C major chords were used to create the theme of piano sonata No.21. If you listen to his symphony No. 5, you wouldn’t expect he was the composer who wrote the piano concerto No.4.
The struggles inherited in Beethoven’s music also make him the greatest. On one hand, Beethoven learned to stand up on his own when he was a teenager. On the other hand, he struggled for personal and artistic freedom on every single level. There is no coincidence that his music was brave, powerful and uncompromised because of his life. His symphony No. 3, No.5 and No.9 are never short of brilliances. One could hear that he was changeling his fate in music against deafness.