When it was noted, with the reforms of the postal service in the 19th century, that the weight of the correspondence was a much or even more important than the distance to be travelled, practical and sensitive scales became de rigueur in all Post Offices. Correspondence is not normally very heavy, but weighing procedures, upon which the cost depends, requires the use of scales of considerable range, i.e. the relationship between the heaviest weight that it is capable of handling and the smallest weight difference it can register, as well as a sufficient degree of sensitivity i.e. it must be capable of determining the weight of the lightest of objects.This Letter Scales was used in the 19th century and was invented by the Sub Inspector General of the Kingdom Posts, Lourenço António de Araújo.This scale was invented to weigh letters, mostly the foreign ones, and initially created to facilitate this tedious and time-consuming task, since it requires a lot of time and many workers, that would result in several errors.The Letter Tax was calculated based on the weight and distance. With the creation of this scale, it was possible to weigh much more quicker and give them the correct tax. Therefore, the great invention about this scale was not the measuring of the weight, but the correct indication of the Tax for each type of letter.