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In a range of hills, or especially of mountains, a pass (also gap, notch, col, saddle, bwlch or bealach) is a lower point that allows easier access through the range. On the route through the range, it is locally the highest point on the route. 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 (the elevation as a function of two position coordinates is mathematically a saddle point). 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 above mean sea level.
There are thousands of named passes around the world; some are familiar names, such as the Great St. Bernard Pass in the Alps, the Khyber Pass between Afghanistan and Pakistan, and the Khunjerab Pass (15,520 feet) between Pakistan and China.
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