BBC TWO (or BBC2 as it was formerly styled) was the second UK television station to be aired by the BBC. It started broadcasting on April 20 1964 during which it suffered a blackout caused by a major power failure. Unlike the other channels available at that time (BBC ONE and ITV) it was only broadcast on the 625 line UHF system, so was not available to viewers with 405 line VHF sets. This created a market for dual standard receivers which could switch between the two systems. BBC ONE and ITV later joined BBC TWO on 625-line UHF but continued to simulcast on 405-line VHF until the early 1980s. BBC TWO became the first British channel to broadcast in colour in the summer of 1967, using the PAL system. BBC ONE and ITV simultaneously introduced PAL colour on UHF in late 1969.
The Beeb's new shows often appear on BBC TWO, especially if those behind the show have not proven themselves elsewhere. A successful BBC TWO show may be moved to BBC ONE, such as happened with Have I Got News For You. BBC TWO's programmes always had a "highbrow" image, compared to those on rival channels. This perception persists in today's multichannel world so that a programme that is moved from BBC TWO to BBC ONE will often get a much larger audience, even though no other change has been made.
BBC TWO itself has been accused of "dumbing down" recently; since the launch of the new digital-only BBC FOUR, the BBC has been accused of letting its more highbrow output go to the new channel (which many viewers cannot receive) instead of BBC TWO, the perceived reason being to allow BBC TWO to show more popular programmes and get higher ratings. Certainly there does seem to be a strong resemblance between the new BBC FOUR and the early, slightly stuffy, BBC TWO.
See also: List of British television channels