Fare construction is a complicated task because each fare comes with a lot of rules regarding the usage, however the rules are designed to be validatable by computers so the system can decide to accept or reject easily. Most commonly fare construction is done by a computer automatically, but it is not guaranteed that the lowest fares can be found. Due to the above reason, or in case when the automatically generated fare has some restrictions which the traveller does not want, manual fare construction can also be done, which means finding the suitable fares manually and applying the fares to an itinerary in order to buy a ticket.