The guide block turns the rope under the water guiding back into the bottom of the rising pipe (main). The guide limits what diameter well or borehole the pump can fit down. More importantly, the part where the rope turns over guide is the main source of wasted energy (inefficiency) in the rope pump and also the point that can experience the most wear. Glass and glazed ceramic two very good materials to use for the turning point as they are very hard wearing and smooth however wood, stainless and galvanised steel are also used. 

The guide must also counteract the pull of the rope to keep the rope tight enough such that it does not slip over the pulley. For this reason the guide is often cast in concrete, especially in open wells where it mus also be kept from moving sideways.