Specific types of pruning may be necessary to maintain a mature tree in a healthy, safe and attractive condition. Crown cleaning is the removal of dead, dying, diseased, crowded, weakly attached and low-vigor branches from the crown of a tree. Crown thinning is the selective removal of branches to increase light penetration and air movement through the crown. Thinning opens the foliage of a tree, reduces weight on heavy limbs, and helps retain the tree's natural shape. Crown raising removes the lower branches from a tree in order to provide clearance for buildings, vehicles, pedestrians and vistas. Crown reduction reduces the size of a tree, often for clearance for utility lines.