Traditional or trad climbing involves rock climbing routes in which protection against falls is placed by the climber while ascending. In the unusual event bolts are used, these are placed on lead (usually with a manual drill). More commonly removable gear called cams, hexes, and nuts are placed in constrictions or cracks in the rock to protect against falls (in place of bolts) but not to aid the ascent directly. Due to the difficulty of placing bolts on lead, bolts tend to be placed farther apart than on many sport climbs. Once bolted on lead, if repeat ascensions can repeat the route using only the previously placed bolts for protection, the route would then be considered a sport climb, and repeat ascents would be considered to be done in the sport climbing rather than trad climbing style. Routes which are protected by a mixture of preplaced bolts and traditional climbing protection (cams/nuts/hexes) are commonly referred to as "mixed" routes, as in a mix of trad and sport climbing. Historically, pitons (a kind of deformable a nail) were placed in constrictions in the rock instead of hexes, nuts and cams. These are difficult to remove and often destructive, resulting in a number of unremovable "fixed" pitons on many older traditionally protected routes. These are frequently used in a similar fashion to bolts, although they are not as trustworthy and by convention are not considered when evaluating if a route is a trad climb, sport climb or mixed climb the way bolts might be.