A door way is a bridge between two rooms, a tunnel a bridge between two lands, a plane is a bridge between two places, a computer a bridge between two people. The bridge is one of mans most important resources and achievements. Cities have been designed around bridges for social, political, and economical reasons. The act of compromising, the bridge between two individual ideas, separates humans from the rest of the animal world. We don't attack other humans that enter our territory or tell us ideas.
Bridges are the link that allows transfer of objects or ideas from place to place.
The places that the bridge touches are the mediators. Each bridge holds its own relationship between the area or ideas that are being crossed. Some relate others don't. If the crossing is an idea or language barrier, then the bridge almost always relates to the crossing at hand, actually incorporating it into the bridging. Where as sometimes a physical bridge will tends to shelter people from the crossing, allowing only subtle notions of the gap.