The presence of the opportunity for casting a single vote (plurality rule) supports two-party systems where the system of proportional representation gives rise to multiple parties in a particular area. This law helps in defining the relationship between the electoral system and the party system that exist in the countries.
Diagrams
Originated from/Coined by
Maurice Duverger
Further Reading/ References
http://www.oxfordreference.com/view/10.1093/oi/authority.20110803095737871
https://www.csmonitor.com/USA/Politics/Politics-Voices/2016/0809/Speaking-Politics-term-of-the-week-Duverger-s-law