In a mountain range, a pass or col is a lower point that allows easier access through the range. Since many of the world's mountain ranges have always presented formidable barriers to travel, passes have been important since before recorded history, and have played a key role in both trade and war.
Topographically, a pass has the general form of a saddle between two mountains. They are often found just above the source of a river, constituting a sort of "bridge" over to the headwaters of a different river. Passes may be very short, consisting of steep slopes to the top of the pass, or valleys of many kilometers, whose highest point is only identifiable by surveying.
The top of a pass is frequently the only flat ground in the area, a high vantage point, so it is often a preferred site for buildings. For countries whose borders are delimited by a mountain range, the pass is typically part of the border, and the facilities likely include a border control or customs station, and possibly a military post as well. For passes with roads, it is also customary to have a small roadside sign giving the name of the pass and its elevation.