Duverger's law

Effect of Duverger's law in UK. The graph shows the vote share of each political party since 1832. Around 1920 a third party (Labour, red) displaces one of the two major parties at the time (Liberal, yellow). After 1980 several third parties build local strongholds and reduce the vote share of the two major parties.

Elections with single-winner ranked voting show the effect of Duverger's law, as seen in Australia's House of Representatives.

There are also cases where the principle appears to have an effect, but weakly: