For many runners, the wall, the 20-mile point when energy tends to sag precipitously, defines the marathon.

But without the wall clearly defining our side and the enemy's, all sorts of lines are being crossed.

Scruffy corrugated ceilings and translucent sliding walls define the common areas.

The building's walls, its idiosyncratic shape, define it as a place apart.

It's beautiful, and its wall literally defines the beginning of the ocean.

The wall would survive for another 1,600 years and define London's perimeter for centuries to come.

These walls define the inner courtyard, and were surmounted by a walkway on which you could walk up to the guard.

The "wall and pier trim" defines a folded plane by continuing around corners.

A dark wall with an open door defines the right end of the mural.

The walls bore brilliant paintings and sharply defined sculptures.