For centuries the ultimate emblem of Britishness has meant different things to different people, and now it is back in the news. What does its history tell us?

Although centuries old, the Union flag – commonly known as the union jack – which represents the unification of England, Scotland and Ireland, is rarely out of the UK news at present. Previously the flag was mostly found displayed on the flagpoles of official buildings but is now being hung from windows of people's homes, and dangled from motorway bridges and in other public spaces.

Its meaning and symbolism are under the spotlight in debates often producing more heat than light. Is the increasingly widespread public display of the union jack – and the St George flag – patriotism or provocation? The argument rumbles on – but what is clear is that the Union flag stands for very different things to different people in different contexts. For some it is simply a symbol of the UK, while for others it is an emblem of Empire, or has unsettling connotations of appropriation by the extreme right.

Professor Nick Groom is a cultural historian whose book The Union Jack: The Story of the British Flag traces the flag's history. He tells the BBC: "In the 1970s, you had [far-right nationalist party] the National Front attempting to appropriate the union jack but you also had Tim Brooke-Taylor of comedy group the Goodies wearing his union jack waistcoats. You had the Queen's silver jubilee, and you also had punks cutting up the flag and turning it into a fashion item.

"There are periodic outbreaks of flag-waving and displays of the flag, some of them endorsed by government, such as happened in 2007 when Gordon Brown became prime minister, and in his first statement to the Commons recommended that government buildings should fly the union jack every day in order to encourage a sense of British identity."

"I think it is incumbent on everyone not to let political extremists set the agenda," says Professor Groom. "If they try to set the agenda, you take it back. This is a flag of inclusivity and diversity. The union jack is almost like a history or map of these isles. Every element has had to compromise to fit in with the others and it has a very long and varied history."

There are three components to the flag. The red cross of St George on a white background is the symbol of England. The diagonal cross of St Andrew on a field of blue is the emblem of Scotland. The red diagonal Irish cross of St Patrick on a white field is the third element.

When James VI of Scotland acceded to the throne of England in 1603 as James I, it created a Union of the Crowns, uniting two kingdoms under one monarch but leaving them constitutionally separate. British naval vessels were required to fly both the red cross of St George and the blue saltire of St Andrew. However, two flags flying from the same mast signified that a military engagement had taken place, with the victor's flag flying above that of the vanquished (according to some, the practice of flying a flag at half-mast means the invisible flag of death is flying above it). English ships flew the cross of St George uppermost, Scottish ships the cross of St Andrew.

This was hardly in the spirit of union that the king wished to encourage, so he had the Earl of Nottingham consider how the two flags might be combined. The earl – who was also Lord High Admiral – favoured a design with the cross of St George alongside the cross of St Andrew. However, according to the protocols of heraldry, whichever flag was nearest the flagpole was superior, so this design did not solve anything.

In 1606, an ingenious compromise was adopted. The cross of St George would be imposed over the cross of St Andrew but the "canton" – the upper portion of the flag nearest the flagpole and, according to heraldry, the most important sector – would be dominated by the colours of St Andrew. Neither insignia had the upper hand. The new "Union Flag" was to be flown by all merchant and royal vessels. There were still unhappy Scots and English........

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