[1] The CQ station code was originally used by land line and undersea cable telegraphy operators in the United Kingdom.
[2] An early land line reference to CQ is contained in Electrical Tables and Formulæ: For Use of Telegraph Inspectors and Operators (1871), which lists "The call for 'all stations' (C Q)...".
[1] Another common qualifier is CQ TEST DE, followed by the sender's station ID, which is used in ham radio contests.
When using CW mode, unattended receivers connected to a computer may automatically decode the signal and log it on the Reverse Beacon Network.
Unlike FM mode, in the SSB voice and CW mode areas of the amateur radio bands operators are free to center their transmissions where it is most optimal (such as away from adjacent traffic that can interfere) and not expected to use whole-number, divisible-by-five, or otherwise channelized center frequencies.