Following a change in the law passed in January 2008, seats will in future be allocated according to the Sainte-Laguë method.
According to Section 6 Paragraph 1 Sentence 2 of the Federal Election Law [de], the second votes of those voters who voted with their first vote for a successful candidate who was either not nominated by a party that also ran on a state list or (this has only been the case since 2011) was nominated by a party that failed to meet the threshold clause are not taken into account for the allocation of seats.
This regulation is intended to prevent these voters from exerting a de facto double influence on the composition of the German Bundestag.
In the case of the FDP, it is significant that during the social-liberal coalition, 29.9% of its second-vote voters supported the SPD direct candidate with their first vote in 1976 and 35.5% in 1980, while in the first election after the coalition change in 1983, 58.3% supported the CDU/CSU candidate, and the figure showed great fluctuations in subsequent elections.
The Greens' second-vote voters gave their first vote in this election to the SPD by 34.4%, and to the Left by 15.7%.