Votes cast in the district also counted towards the calculation of proportional representation ("plurinominal") deputies elected from the country's electoral regions.

[3] It was dissolved in the National Electoral Institute's 2017 redistricting process because the state's population no longer warranted 21 districts.

[4] In its final form, the 21st district's head town (cabecera distrital), where results from individual polling stations were gathered together and tallied, was the city of Cosoleacaque.

It covered 11 municipalities in the state's southernmost Olmeca region: Cosoleacaque, Chinameca, Hueyapan de Ocampo, Jáltipan, Mecayapan, Oluta, Oteapan, Pajapan, Soconusco, Soteapan, Texistepec, Tatahuicapan de Juárez and Zaragoza.

[7][6] The districting scheme in force from 1978 to 1996 was the result of the 1977 electoral reforms, which increased the number of single-member seats in the Chamber of Deputies from 196 to 300.