For extremely slippery or steep snow, ice, and mixed rock and ice terrain climbers must use more advanced techniques, called ice climbing or mixed climbing. Specialized tools such as ice screws and ice picks help climbers build anchors and move up the ice, as well as traditional rock climbing equipment for anchoring in mixed terrain. Often, mountaineers climbing steep snow or mixed snowy rock terrain will not use a fixed belay. Instead each climber on the team will climb at the same time while attached to anchors, in groups of two. This allows for safety should the entire team be taken off their feet which also allowing for greater speed than the traditional technique of belaying one climber at a time. This technique is known as "simul-climbing" or a "running belay" and is sometimes also used on ice, however the risk of dropping frequently displaced ice on the lower team member(s) limits its usefulness on ice. Traditional belays are also used; in this case, this is sometimes necessary due to ice fall hazard, steepness, or other factors.