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 leather or plastic outer shell with a removable insulated 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.