Amateur radio enthusiasts take pride in their ability to send to and receive transmissions from distant locations. Since radio transmissions are transitory, they needed a way to document that these long distance communications occurred. The QSL card was developed in the 1920s to do this.
A QSL card is usually a post card that includes at least the following information: (1) the call sign of both stations, (2) the time and date (usually using UTC), (3) the radio band used, (4) the mode of transmission and (5) a signal report.
The name “QSL card” comes from the Q code “QSL”. Q codes are a type of shorthand used in radio transmissions. QSL means “I acknowledge receipt of your transmission.” As a question, QSL? means “Can you acknowledge receipt of my transmission?”
QSL cards are frequently customized to highlight information about the sender or where the sender lives. Industry publications included the names and call signs of hobbyists looking for connections and willing to confirm communication with a QSL card.
Thousands of different QSL cards have been produced. A few of these have been made on paper money and are interesting crossover collectibles for these two hobbies.