Danny Tan asks:

>Why is Mozart's work starts with K--- instead of Op---? Are there any
>particular reason why he is special?

Mozart didn't get around to numbering his works until 1784, when he had
already completed all but 176 of his more than 600 works.  Ludwig Koechel,
who was an Austrian devotee of Mozart's music (as well as a botanist and
mineralogist) compiled a chronological and numbered catalog of Mozart's
works, with the first few bars of each, and published it in 1862.  There
were several later editions of this by musicologists including Alfred
Einstein.  (Information from the New Grove.)

The music of Bach, Scarlatti and Schubert, and no doubt others who don't
come to mind) is also numbered by non-opus numbers.  The first listing
of published music by opus numbers goes back to the 16th Century.

Jim Tobin