In Ham Radio, SK has a dual meaning. When you see it place after a callsign, SK stands for “silent key,” signifying the operator is deceased. SK is also a CW “prosign” that means “end of contact. It is used at the end of a QSO before the callsign. From the ARRL’s glossary of terms: “In the Western Union company’s ‘92 code’ used even before the American Civil War, the number 30 meant ‘the end. No more.’ It also meant ‘good night’. In Landline Morse, 30 is sent didididahdit daaah, the zero being a long dash. Run the 30 together and it has the same sound as SK.”