Barbed wire used to do it, corrugated iron decades later, but they both make barriers of a hasty and panicky appearance.
Makeshift cordons are much the same the world over, the inhumanity of human to human a shaming spectacle everywhere. No wonder a well-intentioned Trina Vargo (Opinion & Analysis, February 5th and in a similar essay last September) wants people in Belfast to take down at least a section of the misnamed "peace wall". To strengthen her argument, she has the results of a specially commissioned poll to show that a majority of residents near some walls would like them down - though most, she adds honestly, say not yet.
American positive thinking helped hugely to propel the North towards peace. Take a long view and it is clear the advance began in earnest when British pessimism and Northern near-despair opened up to the light of outside interest, much of it enlisted by the strategic and visionary optimism of John Hume. The advocacy of someone such as Vargo is worth hearing. She made her career, after all, as foreign policy adviser to Ted Kennedy and has dared to say Irish illegals in America deserve no special treatment.
It takes fresh eyes and minds to suggest that 40-foot high fencing between Catholics and Protestants and brick walls across streets need not be permanent structures. But it takes no more than a well-informed drive in and around them to remember why each is where it is, and to see that anyone who lives elsewhere would have some nerve persuading the residents to take up their chisels and set about peaceful demolition.
A top dressing of normality may hearten many, but in dozens of streets people know that one bad injury from a well-aimed stone could still bring out hysterical crowds. They live along the faultlines of an ancient quarrel unresolved, indeed largely untouched, by today's peace.
Soldiers fresh off ships from England in August 1969 into the smoke and terror between Falls and Shankill stretched out coils of spiky wire, too late, much as their predecessors did in the 1920s Troubles almost half a century earlier.
"Rolls of barbed wire fenced off Ardoyne and the Bone," as historian Jonathan Bardon wrote of embattled north Belfast Catholics in 1922. That first barbed-wire "peaceline" of the most recent Troubles went up where rioting began in 1886. Much the same emotions maintain separation as in 1835 and 1935. Only the cloth-eared say people are "separated" by walls.
But then it also stretches the facts to bestow the title of peaceline, and the even more grandiose "peace wall" on multiple barriers, sometimes between one half of a street and another. Barricades thrown up at the worst moments have been made of anything likely to repel or delay a mob or shelter a sniper. Today's peacelines in Belfast are as different as the new balance of populations.
Big, young Catholic communities to one side, ageing and much smaller Protestant communities to the other, most have modernistic structures of brick and sometimes landscaping between them. The structures are no more unified, or uniform, than the people. Some have little gates built in, with permanent notices announcing opening times.
There the peacelines are, and there they stay, to the embarrassment of some on both sides of them, utter indifference from Northerners who live elsewhere, the academic engagement of many, the entertainment of tourists, and the benefit of visiting journalists who relish a story that is also a photo opportunity they can drive right up to and use as backdrop.
Since paramilitary violence has ebbed and some kind of normality has begun to emerge, the most makeshift remaining walls have been tidied up or rebuilt.
As populations spread or shifted some demanded, and got, new walls. In the decade after the first IRA ceasefire, the kind of clashes that have happened for more than two centuries recurred but this time drew unusual interest - since now there was meant to be peace. The background static of low-level violence became audible because the bombs and most of the guns had fallen silent.
Cue discovery that reconciliation had not been part of the "peace process", and sociological papers demonstrating that there had been an increase in segregation, sectarianism and even in violence - although the killing had all but stopped.
The organisation that Trina Vargo founded and leads, the US-Ireland Alliance, polled three districts in December to see what support there was for removing a barrier this April, to mark the 10th anniversary of the Belfast Agreement.
Vargo admits she now thinks it unlikely that there will be a partial dismantling, which would have been a high point of the anniversary conference she has planned - to be held in Queen's University, not at a flashpoint. The people who live either side of walls need leadership to build confidence, she says, and the issue needs debate.
She has a point.