QSL stands for “I acknowledge receipt,” or “I hear you.”  It comes from the Q codes, a set of radio shorthand developed by the British government around 1909, which were in turn based on similar codes developed by telegraph operators.  When radio amateurs started communicating a few years later, they wanted to provide proof of their contact with other amateurs.  This proof quickly became the QSL card: a postcard verifying contact between two stations.  When broadcast radio stations sprang up, the hobby of long-distance radio listening, or DXing, began too, and these stations started issuing QSL cards as well.  QSLs have been issued as cards (the most common), letters, stamps or pennants; there are even a few examples of QSLs printed on balsa wood or sheets of copper.

QSL cards fall into three general categories, and the easiest way to understand the differences is to divide the radio world this way: amateur radio operators, radio stations, and listeners. 

Ham or amateur radio QSLs are for contact between two amateur radio operators. The cards are proof that the two operators have been in touch with each other.  Hams will list the date, time, and frequency of the contact with each other, usually with information about their equipment.  A ham card will always list its call sign.  Here is a typical US ham card; this one is from the 1930s.



Radio station QSLs are sent to listeners who have sent a report to the station requesting verification of reception.  The cards are proof that the listener has heard the station.  Here are both sides of a 1970 card from Radio Japan.

After World War II, most shortwave stations identified by a station name; earlier, the use of call signs was common.  In the United States, shortwave stations of the 1930s used an experimental call sign with an "X" in the third position: W1XAL or W4XB, for example. 

CB (Citizen's Band) QSLs were designed for the same use as amateur cards.  Citizen’s Band is a set of frequencies designed for local use, with minimal requirements for getting on the air.  SWL cards (for Short Wave Listener) were designed to be sent to amateurs by listeners.  Most CB and SWL cards, however, were simply swapped as a hobby.  In the United States, CB cards can be identified by the call sign: Three letters followed by four numbers.  In Canada, CB calls begin with "XM," followed by five numbers.  SWL cards usually use some form of “SWL” for the call, or a sign that looks like a ham sign, but has two or more numbers. These can be distinguished from ham cards by the fact that they indicate that the station was heard, not contacted.  SWL cards are mostly from the 1930s; CB cards are mostly from the 1960s and 70s.   There is a less common type of swapping card from the early 1960s, the WPE card, using call signs beginning with “WPE,” issued by Popular Electronics Magazine to radio listeners.  The card on the left is a typical CB card of the mid-1960s; the card on the right is an unusual WPE/CB combination card.



There are a few special areas in QSLs, just like any other hobby.  EKKO stamps, manufactured by the EKKO Company of Chicago, were sent as verification of reception by a number of US, Canadian and Cuban stations in the 1920s and 30s; some stations sent similar stamps of their own design.  The Bryant Company, also of Chicago, issued a similar group of stamps, adding The Philippines, Great Britain and Mexico to its list of stations. There is an excellent guide to EKKO stamps by czelbst here.  A similar guide to Bryant stamps is here.  Here are examples of EKKO and Bryant stamps.

 

Radio station QSLs divide into a few categories:

• Shortwave stations, which are on various bands of frequencies from about 2500 kilohertz  (2.5 megahertz) to about 26000 kilohertz (26.0 megahertz), are able, with sufficient power, to broadcast worldwide. 
• Medium Wave, or AM radio stations, which are on frequencies from 540 kilohertz to 1700 kilohertz, usually are heard only locally, though international DXing at these frequencies is possible.  Higher frequency stations such as FM or TV stations have issued similar QSLs.
• Occasionally, utility station QSLs are available.  These stations are transmitting from one point to another (point-to-point), usually to complete telephone calls or to relay programs for rebroadcast.  Included in this category are time stations, government-operated stations broadcasting time and technical measurement information.

In the Ham Radio QSL world, there are also some specialty areas.  The ham radio world has a different definition for “country” than, say, the United Nations does.  It’s complicated, but it means that many small out-of-the-way locations are considered countries for DX, or long-distance contact purposes.  Some countries no longer exist (Czechoslovakia, for example, which is now two countries, the Czech Republic and Slovakia); they’re called deleted countries.  Others are uninhabited, which means QSL cards are only issued when amateurs travel to them in a DX-pedition.  A few amateurs such as Danny Weill, Gus Browning, and Lloyd and Iris Colvin traveled worldwide in DX-peditions and issued distinctive cards from each country where they transmitted with special call signs. There is an additional specialty involving contact with islands called Islands on the Air (IOTA).  

What’s a QSL card worth?  Prices can range from pennies to thousands of dollars.  There is no real guide to prices; in fact, there’s no real inventory of what’s available.  A card may have value because it’s rare (for example, Diu, a deleted country, had only one amateur, and around 50 QSL cards issued; these cards are quite valuable); a card may have value because of its historical context (cards from Spanish broadcast stations during the Spanish Civil War are valuable in part because these stations were the first to use radio and QSL cards as propaganda tools).  The best guide is to follow the eBay listings to get a feel for prices.

A number of QSL card owners have posted parts of their collections on the Internet.  There is also a collection of mostly broadcast station QSLs housed at the University of Maryland in College Park; selections from their collection can be found at http://www181.pair.com/otsw/cprv-gallery.html.

This last card, by the way, is from the first shortwave broadcasting station, PHI, operated by the Philips Company of the Netherlands.  It began operation in 1927.