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The Great St. Bernard Pass, still snowy even in June, has long been a major route through the Alps.

A mountain pass as it might appear on a contour map: Bwlch Maesgwm in Snowdonia (53°4.88′N 4°7.95′W / 53.08133, -4.1325, height contours from SRTM data).

A photo of the approach to the same pass as in the above map (looking from the NNE).

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 a 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 trade, war and migration.

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.

Roads have long been built through passes, and more recently railways. Some high and rugged passes may have tunnels bored underneath, so as allow faster traffic flow year-round.

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 (2,473 m) in the Alps, the Khyber Pass (1,027 m) between Afghanistan and Pakistan, and the Khardung La (5,359 m) in Jammu and Kashmir, India.

The world's highest motorable pass may be the little known Marsimik La (5,590 meters) in India near the Chinese border. Marsimek-La Pass is on the northern-most tip of the Changthang Plateau.

The word gap is more commonly used in the southern Appalachians, while notch is more common in New England.

In Scotland, the Gaelic term bealach (anglicised "Balloch") is often used.

As well as allowing easier access between two valleys, a pass also similarly provides the route between two mountain tops with a minimum of descent, making it important also to hikers. Because of these advantages, it is common for tracks to meet at a pass, making them often convenient routes even when travelling between a summit and the valley floor.

Argentina and Chile share the world's third longest international border, 5.300 km long, running from north to the south through the Andes mountains. They share 42 mountain passes between them. ^{(see list)}

• List of mountain passes
• Principal passes of the Alps, Ranges, Peaks and passes of the Alps

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Mountain passes