Who painted Guernica?

Picasso, in about a month in 1937, after German and Italian aircraft bombed the Basque town at the request of Franco's forces - a market-day attack on civilians that was among the first demonstrations of what aerial bombing would do to cities, and was watched closely by every air force in Europe.

It is enormous, three and a half metres by nearly eight, and entirely in grey, black and white - which is usually read as newsprint, the medium through which the world learned about the bombing. There is no aircraft in it and no bomb; there is a screaming horse, a bull, a dismembered soldier, a woman holding a dead child and a light bulb in the shape of an eye. Picasso consistently refused to explain the symbols, saying it was for the viewer to interpret.

He would not let it go to Spain while Franco lived, and it spent decades at the Museum of Modern Art in New York before arriving in Madrid in 1981. A tapestry copy hangs outside the United Nations Security Council, and in 2003 it was covered with a blue curtain for the press conference at which the case for invading Iraq was made - the UN said it was for technical reasons about camera backdrops, which nobody believed.