There are three types of mountain climbing. In the easiest, trail climbing, participants merely hike along trails to the top of a particular mountain. The trails generally are not very steep, and the mountains are relatively small. Rock climbing takes place on steeper slopes and larger mountains. Participants generally have to ascend on hands and feet, employing special equipment that may include thick rubber-soled boots or other special shoes, rope, and steel spikes, known as pitons, that are driven into the rock as an aid to climbing. Ice climbing is generally required only on extremely high mountains whose peaks are above the timber line. Equipment used in ice climbing includes the ice axe and attachable boot spikes, known as crampons, that are used on hard ice or snow.