Electoral system for national legislature

1917 Constitution amended up to 2009, Article 54

The election of the two hundred deputies according to the principle of proportional representation, and the system of assignment by regional lists, is subject to the following bases, and what the law disposes:

I. A political party, to obtain registration of its regional lists, must run candidates for deputies by plurality in at least two hundred single districts.

II. All political parties that get at least two percent of the total of the vote cast for the regional lists of the multiple electoral areas will have the right to have deputies according to the principle of proportional representation.

III. To the political parties that meet the requirements of the above two bases, in addition to the plurality results that their candidates receive, seats will be assigned by the principle of proportional representation, in agreement with the parties' national vote, and the number of deputies on their regional lists that correspond to each multiple area. In the assignment, the order that they candidates had on the corresponding lists will be followed.

IV. In no case may a political party have more than three hundred deputies by both principles.

V. In no case may a political party have a number of deputies by both principles that exceeds its percentage of the national vote by eight percent. This base will not apply to the political party by which, through its victories in single election districts, obtains a total number of seats in the Chamber that exceeds the percentage of its national vote plus eight percent.

VI. In the terms established in sections III, IV, V, and VI above, deputies of proportional representation that were assigned according to what is disposed in sections V or VI who leave office, will be replaced by these political parties with one of their own from the multiple areas, in direct proportion to the national vote that the parties received, in the last election. The law will develop the rules and formulas necessary for these effects.

Reforming Electoral Systems in Mexico, Horcasitas and Weldon, in Shugart and Wattenberg, Mixed Member Electoral Systems, OUP 2001, pp209-230 (esp. table pp228-229)

This is how International IDEA traditionally classified the 1996 system.  The seat allocation is in fact parallel, but with two constraints: no party can win more than 300 seats, and no party can win more seats than would be justified by it polling 8% higher than it actually did in the List PR section.  (Thus an overall majority can only be achieved by polling 42%).

300 FPTP: 200 List PR using closed list and largest remainder