The stiffness of the boot enhances the precision of the crampon and allows a climber to pursue steeper and more difficult terrain.
Double boots, another classification, typically consist of a non-insulated leather or plastic outer shell with a removable insulated liner.
Since the insulation cannot be removed from single boots, they are not ideally suited for multi-day cold weather excursions.
They are not completely stiff like ski boots as they need some degree of flexibility for activities such as hiking and snowshoeing.
In recent years, companies have been making mountaineering boots to be NFPA compliant so they may be used in wildland firefighting.