http://www.electionworld.org/germany.htm
The election results in the link are from the 2002 election. In that election the reds (SDP) and the greens (Green Party) joined together to form a coalition government.
To form a government in the Bundestag (German Federal Diet - Parliament), your party or coalition needs 302 seats out of 603.
“Official results showed Merkel’s party (CDU/CSU) winning 225 seats, three more than the Social Democrats (222 seats). The Free Democrats got 61, the Greens 51 and the new Left Party, an alliance of ex-communists and former Social Democrats alienated by Schroeder’s efforts to trim the welfare state, 54.”
http://www.cbc.ca/cp/world/050919/w091912.html
Usually the CDU/CSU joins with the Free Democrats to form governments when the CDU/CSU wins. But together, the CDU/CSU and FDP have only 286 seats, short of the 302 seats needed.
Usually the SDP and Greens join together to form governments when the SDP wins. But together, the SDP and GP have only 273 seats, short of the 302 seats needed.
Once or twice (maybe more) the Free Democrats joined with the SDP to form a government. But this election the Free Democrats vowed not to join the SDP. They said there would be no “traffic light government” [Reds (SDP), Yellows (FDP), and Greens[/color]].
Also, you may be wondering why doesn’t the SDP join with the greens and Left Party to form a government? Because the SDP said they will not form a coalition government with them. They do not want to give the far leftist any concessions, as parties do when they form coalition.
So the other options: CDU/CSU join with the SDP to form a government. CDU/CSU join with the FDP and “The Greens”. The later is not very likely. The former has been mentioned, but that is speculation.