People such as taxi-drivers who travel a lot through the streets of Belfast use a peculiarly local type of satellite-navigation system to take their bearings about where they should be going.
This important device does not sit on the dashboard of their cars. Instead they carry it in their heads: the cumulative knowledge of decades of experience of where's Protestant, where's Catholic, where's safe and where's dodgy.
Most often people do not speak openly about such things: they don't have to, since it is just one of scores of ways people use to get through life in a divided city.
By this stage, after four decades of the Troubles, it is simply second nature: that's life in Belfast.
The city is criss-crossed with dozens of tall metal and brick peace lines, towering structures erected to keep the two sides apart.
Some of them are familiar, having been in place for more than 30 years, unmistakeable signs of division. Yet Belfast society is also fitted with a formidable array of other peace lines which are less conspicuous.
They exist in education, leisure, sport and dozens of other facets of life, permeating society and, arguably, constituting a more important division than class. It has long been acknowledged that the cultural cost is high. Now, the financial implications have been laid bare.
The differing attitudes of nationalists and unionists is illustrated by the largest parties on each side: the first vote in the main for Sinn Fein, while the second supports the Rev Ian Paisley.
Their widely differing takes on the Troubles were starkly illustrated by a poll that showed 86 per cent of Protestants approved of the police using plastic bullets while 87 per cent of Catholics disapproved.
The gulf in these mindsets is so wide that, apart from television and radio debates, it is extremely rare for committed unionists and committed nationalists to debate such things.
British governments would prefer a much greater element of mixing in Northern Ireland society, but there is much self-segregation in Belfast, with a majority of the population accepting they will live parallel lives.
This does not mean that everyone in Belfast is a bigot: a great many have friends across the divide.
But most just accept that remaining on one side or the other makes life less complicated and safe. Those peace lines, both visible and invisible, are daunting barriers to progress in the future.Reuse content