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The extra height and stiffness of mountaineering boots helps support the climber in steep terrain where flexible boots could cause unsure footing and possibly result in a fall. This extra stiffness is traditionally achieved through the use of a full steel shank, though some manufacturers have begun to use carbon fiber to create the necessary stiffness.
Mountaineering boots are typically designed to be used with crampons. To achieve compatibility with crampons, welts are molded into the toe and heel of the boot, providing a platform for the crampon to attach to. The stiffness of the boot enhances the precision of the crampon and allows a climber to pursue steeper and more difficult terrain.
Another important aspect of mountaineering boots is warmth. To prevent frostbite in extreme cold conditions, synthetic insulation is incorporated into the boot. Lighter weight boots are classified as single boots. These single boots are insulated as a single unit with the insulation being permanently attached to the rest of the boot. Double boots, another classification, typically consist of a non-insulated shell with a removable insulated liner. They have either a leather or plastic outer-shell as well as a removable liner. The purpose of a double boot is long term protection from cold weather. If the liner becomes wet, it can be removed from the shell and dried inside a sleeping bag. Since the insulation cannot be removed from single boots, they are not ideally suited for multi-day cold weather excursions.
Other uses 
Mountaineering boots are used for ice climbing, mixed (rock & ice) climbing, and crevasse traverse and rescue. They are not completely stiff like ski boots as they need some degree of flexibility for activities such as hiking and snowshoeing.
The versatility of these boots also makes them popular for other activities. With their durability and extreme-weather adaptability, they are worn by snow-plough drivers, cable car operators, mountain rangers (especially in the winter), rescue paramedics, ski lift operators and snowcat drivers.
Mountaineering boots are generally considered to be too stiff for backpacking on established trails.
- Mountaineering Boots - Spadout