The study compares the surnames from the Tax Lists from Bohemia (1654) and Moravia and Silesia (1669‒1679) with those from the current surname list (2017), with the aim to map the changes of the onymic system. A special attention is paid to changes in the names beginning with the letter K, with a particular focus on surnames which have already become extinct or they are about to disappear soon. The reasons for the disappearance of surnames are primarily biological (no sons in the family). However, some family names have been changed over the years, and another reason is the migration, which has had an impact on the onymic system as well. The system of Czech surnames underwent great changes as a result of the Second World War, when some Jewish surnames disappeared and the surnames of the expelled German inhabitants were lost. The article also poses a question which surnames can actually be considered to be Czech.