CWIn the HamQSO filter: CW
CW: what it is and where to find it on the bands
CW, Morse code by radio: the narrowest, most efficient mode, and still where most rare DX is worked.
CW is Morse code sent by switching a carrier on and off. It needs very little bandwidth and gets through when voice cannot, which is why DXpeditions, QRP stations and SOTA activators lean on it.
On a cluster, CW spots often come from the Reverse Beacon Network, skimmers that decode CW automatically and report the speed in words per minute together with the signal-to-noise ratio.
You will find CW at the bottom of every HF band. On 30 metres it shares the band only with the digital modes.