The Relationship between municipalities and their codes by provinces contains the relationship of all municipalities according to the denomination registered in the Register of Local Institutions (REL) of the Ministry of Territorial Policy, being the National Statistics Institute the institution responsible for assigning the corresponding codes. This relationship is updated every year with reference to 1st January and, in addition, amendments arisen over the course of the year are also published, as they are reported through the Register of Local Institutions.

The municipal code is made up of five digits: the first two correspond to the province code and the remaining three to the municipality code within it. Furthermore, a sixth control digit is published, assigned through a calculation rule, that facilitate the detection of recording and coding errors.