[1] It elects one deputy to the lower house of Congress for each three-year legislative session by means of the first-past-the-post system.

Votes cast in the district also count towards the calculation of proportional representation ("plurinominal") deputies elected from the fourth region.

The 12th district thus covers Mexico City's Centro Histórico.

[5][1] Under the 2005 districting scheme, the 12th district covered that portion of the borough (delegación) of Cuauhtémoc to the south and east of Paseo de la Reforma and Calzada de Guadalupe.

[6] Between 1996 and 2005, the district covered the southern and western portions of Cuauhtémoc.