Used in their formal "question/answer" sense, their meaning varies depending on whether they are sent as a question or an answer. For example, the message "QRP?" means "Shall I decrease transmitter power?", and a reply of "QRP" means "Yes, decrease your transmitter power". This structured use of Q codes is fairly rare and now mainly limited to United States amateur radio CW traffic networks.
In modern everyday amateur radio practice, the Q codes are more commonly used as shorthand nouns, verbs, or adjectives. For example, one will sometimes hear a ham complaining about QRM or telling another ham that he "has QSB on his signal"; if a ham wants you to change your operating frequency, she will ask you to QSY. Although the Q codes were created for use during Morse code operation, they are now commonly used in voice modes too. The following table gives the most common Q codes used in the amateur service, along with their meaning and sample use.
|QRL||Is this frequency busy||Used almost exclusively with Morse code|
|QRM||Man-made interference||There's another QSO up 2 kHz that's causing you a lot of QRM|
|QRN||Static crashes||The band is noisy today; I'm hearing a lot of QRN|
|QRP||Low transmitting power||I'm using a QRP transmitter here, running only 3 watts|
|QRS||Send your Morse code more slowly||Please QRS, I'm new to Morse code|
|QRT||Stop sending||I've enjoyed talking to you, but I have to QRT for dinner now|
|QRV||Active; in use||Will you be QRV in the upcoming contest?|
|QRX||Hang on a minute, I'll be right back||Please QRX one|
|QRZ||Who is calling me?||QR Zed? I hear someone calling, but you're very weak|
|QSB||Fading of signal||I'm hearing a lot of QSB on your signal|
|QSL||Acknowledge receipt||I QSL your last transmission|
|QSO||A conversation with another ham||Thanks very much for the QSO|
|QSY||Change frequency||Let's QSY up 5 kiloHertz|
|QTH||Location||My QTH is South Park, Colorado|
|QTR||Exact time||QTR is 2000 Z|
Some of these common usages vary somewhat from their formal, official sense.
Some Q codes are also used in aviation, in particular QNH and QFE, referring to certain air pressures. These codes are used in radio conversations with air traffic control as unambiguous shorthand, where safety and efficiency are of vital importance.
See also: Common Morse code abbreviations in the Morse code article