In 1851, Sir Ludwig von Köchel (botanist and mineralogist) started to create a complete chronological catalog of Mozart's compositions. The daunting task proved harder than it seemed. Although Mozart's life was short, he was extremely prolific. Mozart also wrote many new compositions to stay out of debt that were given away and there were not always accurate records of who ultimately ended up with the compositions. After almost 12 years in 1862, Köchel published the first edition of his catalog, now known as K1.
Count Paul von Waldersee published K2 in 1905, which consisted of the original catalog with minor corrections and additions. The first thorough revision, K3, was published in 1936 by Alfred Einstein. He used lower case letters between the previously established numbers to account for compositions misplaced in the original catalog or previously unknown. K3a was published in 1946. This edition was a reprint of K3 with an extensive supplement of corrections and additions. The fourth and fifth editions consisted of unchanged reprints of K3. The last revision, K6, was published in 1964,. This edition continued Einstein's innovations, added more numbers with lower case letters appended along a few with upper case letters. The seventh and eighth editions are reprints of K6 with no changes.
Given that the catalog has had several revisions, some compositions now have two K numbers and a few have three K numbers. One such example is Mozart's Symphony in B-Flat major, K182=K166c=K173dA.
Also K, KV.