From farm to yard to factory to community, fences define the American landscape. Fences keep people apart. Fences bind people together. Fences are about relationships between people.

"There are fences everywhere; you can't take a walk without running into fences. They seem natural, but they're not; people put them there," curator Gregory K. Dreicer said this week, introducing a new exhibit at the National Building Museum."He surveyed the fence and all gladness left him and a deep melancholy settled down upon his spirit. Thirty yards of board fence nine feet high. Life to him seemed hollow and existence a burden," Mark Twain wrote of Tom Sawyer, viewing a fence he must whitewash.

Yet Robert Frost's "Mending Fences" offers a brighter view with the refrain: "Good fences make good neighbors."

That's certainly the case when fences keep the neighbor's livestock out of your cabbage patch, but fences can also mark bitter bounds, Dreier said, noting giant "spite" fences erected to block out annoying children or pets.

"Fences are weapons, but they are also tools that help people live together," Dreicer said. They let people feel in control of their own space.

Control was the intent of those first fences in colonial America, used as symbols of ownership and pride in property, he said.

When Europeans came to this continent they took the land and used fences as justification, he said.

"Native Americans don't fence the land, so it's not theirs. We fence the land, so it's ours," was their contention, said Dreicer. So, the first thing colonists did when they cleared land was to fence it in.

A New Yorker, Dreicer became fascinated by the zigzag wood rail fences common in the South and began studying the history of fencing. That led to the museum's new exhibit, "Between Fences, " running through Jan. 5.

Visitors pass between fences as they wander through the show, first a dry stone fence like those on New England farms, then the wooden rails that seem to be in every Civil War photo. There are rolls of barbed wire and chain link everywhere, picket fences large and small, wood posts and decorative wrought iron from family cemeteries.

Most of the South was unfenced until after the Civil War, Dreicer said. Only then did landowners decide to close in their property, to prevent the newly freed slaves from grazing stock and planting gardens.

As the West developed, the battles between farmers and stockmen over fencing became deadly, but the barbed wire finally won out, taming the land for the plow.

In the late 1800s chain link - now seen nearly everywhere - came on the scene. Though it may look like continuous wire, visitors can see a German-made machine making it - twisting wires together in a series of linked pairs called pickets in the fence business.

Finally, visitors pass through a giant picket fence to a display of international borders - the steel fence used along parts of the U.S.-Mexico border and the series of pylons that mark the line with Canada.

People, concluded Dreicer, "define their lives, the spaces they live in, through the borders they build."