Electoral districts are identified by Roman numerals and by federal entity (state or the Federal District). The number of electoral districts was set at 300 in 1979, when the number of seats in the Chamber of Deputies was increased from 196. The demarcation of the districts depends on the results of the previous electoral census, and adjustments to the 1979 districts were made in 1996 and 2005.
Irrespective of population, no state may be represented by fewer than two electoral districts. This is the case with Baja California Sur (population: 512,000), Campeche (population: 755,000) and Colima (population: 568,000), which, as a result, return more senators than deputies to Congress. The states with the most electoral districts are the state of México (population: 14 million), with 40, and Veracruz (population: 7.1 million), with 21. The Federal District, with a population of 8.8 million, has 27.