The most commonly used method of removing water vapour from air (dehumidification) is to cool the air below its dew point.
The dew point of air is when it is fully saturated i.e. at 100% saturation.
When air is fully saturated it cannot hold any more moisture in the form of water vapour.
If the air is cooled to the dew point air and is still further cooled then moisture will drop out of the air in the form of condensate.
This can be shown on a psychrometric chart as air sensibly cooled until it becomes fully saturated (the dew point is reached) and then the air is cooled latently to a lower temperature.
This is apparent on the psychrometric chart as a horizontal line for sensible cooling to the 100% saturation curve and then the process follows the 100% saturation curve down to another point at a lower temperature.
This lower temperature is sometimes called the Apparatus dew Point (ADP) of the cooling coil.
In reality the ADP of the cooling coil is close to the cooling liquid temperature inside the coil.
Chilled water or refrigerant may be the cooling liquid.
The psychrometric process from state point 1 to 2 to 3 may be shown as a straight line for simplicity as shown above with a yellow line.