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

[4][5] In its final form, the 20th district comprised 14 municipalities: Cosoleacaque, Chinameca, Hueyapan de Ocampo, Jáltipan, Mecayapan, Oluta, Oteapan, Pajapan, Soconusco, Soteapan, Texistepec, Zaragoza, Nanchital and Tatahuicapan.

The district's head town (cabecera distrital), where results from individual polling stations were gathered together and tallied, was the city of Cosoleacaque.

[9][8] 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.

[10] The newly created 20th district had its head town at Zongolica, and it covered the municipalities of Acultzingo, Astacinga, Atlahuilco, Camerino Z. Mendoza, Coetzala, Cuichapa, Cuitláhuac, Magdalena, Mixtla de Altamirano, Naranjal, Omealca, Los Reyes, San Andrés Tenejapan, Soledad Atzompa, Tehuipango, Tequila, Texhuacán, Tezonapa, Tlaquilpa, Tlilapan, Xoxocotla, Yanga and Zongolica.