本文檢視全世界民主國家在不同總統與國會選制的搭配組合下，政黨體系有何不同。本文發現，就國會選制而言，國會選舉採比例代表制的國家， 其國會有效政黨數在整體上明顯多於國會選舉採單一選區相對多數制的國家;而不論是國會選舉採比例代表制或是採單一選區相對多數制的國家，有總統直選制度之國家的國會有效政黨數，明顯少於無總統直選制度的國家。 進一步言，在國會選舉採比例代表制的國家中，若總統選舉採相對多數制，其國會有效政黨數會少於總統選舉採兩輪決選制的國家;不過，在國會選舉採單一選區相對多數制的國家中，不論總統選舉採行相對多數制或兩輪決選制，其國會有效政黨數的差異不大。

就總統選制而言，總統選舉採相對多數制的國家，其總統有效候選人數在整體上明顯少於總統選舉採兩輪決選制的國家。進一步觀察，會發現在總統選舉採相對多數制的國家中，若國會選舉採單一選區相對多數制，則總統有效候選人數顯得更少。而在總統選舉採兩輪決選制的國家中，若國會選舉採比例代表制，則總統有效候選人數顯得更多。至於總統選舉採相對多數制而國會選舉採比例代表制的國家，以及總統選舉採兩輪決選制而國會選舉採單一選區相對多數制的國家，這兩類國家的總統有效候選人數明顯介於前兩類國家之間。

This study comprehensively observes the party system under different collocations of presidential and parliamentary electoral system in democracies all over the world. Regarding parliamentary electoral system, it is found that the effective number of parliamentary parties in the countries adopting proportional representation system(PR), overall, is apparently larger than that in countries adopting plurality with single-member-district system(SMD), and that in countries holding direct presidential elections is clearly smaller than that in countries not holding direct presidential elections, regardless of which is adopted for parliamentary electoral system. Furthermore, as for countries adopting PR for parliamentary electoral system, the effective number of parliamentary parties in countries adopting plurality system for presidential electoral system is smaller than that in countries adopting majority system for presidential electoral system. However, as for countries adopting SMD for parliamentary electoral system, there is little difference in the effective number of parliamentary parties no matter whether plurality or majority system is adopted for presidential electoral system.

Regarding the presidential electoral system, the effective number of presidential candidates in countries adopting plurality system for presidential electoral system, overall, is smaller than that in countries adopting majority system for presidential electoral system. To explore further, it is found that in countries adopting plurality system for presidential electoral system the effective number of presidential candidates appears smaller if SMD is adopted for parliamentary electoral system. However, in countries adopting majority system for presidential electoral system, the effective number of presidential candidates appears larger if PR is adopted for parliamentary electoral system. As for countries adopting plurality system for presidential electoral system and PR for parliamentary electoral system, and countries adopting majority system for presidential electoral system and SMD for parliamentary electoral system, the effective numbers of presidential candidates obviously lie between those of the above-mentioned two groups of countries.