Wood bars or steel rails fixed vertically to cross buntons in a shaft. The cage shoes travel along the guides and therefore prevent the cage from swinging and doing damage in the shaft. Some skips are fitted with rubber-tired rollers running on 6-in by 4-in (15-cm by 10-cm) steel channel guides. Guide shoes may be fitted to act as alternative guides in 1214 case of breakdown of rollers. Fixed guides are used when shaft space is limited; i.e., when the clearances do not permit the use of flexible or rope guides.