In the illustration to the right we have a rope attached to a load weighing 100kg. The rope has been passed through a pulley which is attached to an anchor point and returns back down to ground level.
The amount of effort required to lift the load in this situation is 100kg so we have not formed any mechanical advantage. This system has a ratio of 1:1, additionally for every metre of rope that the user pulls through the system the load will be raised by a metre.
All this system does is change the direction of where the effort needs to be put in, instead of pulling the rope in an upwards direction it can now be pulled downwards which is usually more efficient. This is commonly referred to as a directional or redirect pulley.
In this situation the directional or redirect pulley and its anchor point will actually be feeling double the weight of the load, as there is the loads weight on one side and an effort of 100kg needs to be applied on the other side to raise the load.