Millions of Germans are casting their votes in the country's federal elections, with Chancellor Angela Merkel tipped to retain power.She is seeking a fourth term in office and to keep her conservative CDU/CSU alliance's status as the largest presence in Germany's Bundestag.Its coalition partner, the social democratic SPD, is its main rival, while the right-wing AfD is likely to gain its first parliamentary seats.Voter turnout is expected to be high.Polling stations opened at 08:00 local time (06:00 GMT) and close at 18:00, with exit polls expected shortly after.The election is seen as important because it may result in six parties in the Bundestag - the German national parliament - for the first time since World War Two.Such a result could mean a change in the makeup of the current governing coalition.Uncertainty about what a new coalition government could look like is expected to draw high numbers of voters to the ballot boxes, with postal voting higher than usual.Key contendersAngela Merkel: Widely expected to retain her seat, Mrs Merkel is seen internationally as a source of stability - having led Germany since 2005. Her decision to open Germany to asylum seekers during the peak of Europe's migrant crisis cost her politically, but she appears to have recoveredMartin Schulz: The SPD leader was until recently speaker of the European Parliament. Mrs Merkel's main rival is also her coalition partner - which has proved troublesome during the campaign, as he sought to criticise her politicsAlice Weidel and Alexander Gauland: The top candidates for the AfD. Achieving seats in the Bundestag - or potentially becoming the third-largest party, as some campaign polls suggested - would be a major victoryWhat are the coalition options?The SPD's Mr Schulz has already appeared to reject the idea of another CDU-led grand coalition, so what are the realistic alternatives for Mrs Merkel? A “Jamaica” coalition is seen as increasingly likely: so-called because of the colours of Jamaica's flag - with the black CDU/CSU, the yellow, business-friendly FDP and the Greens. Not a marriage made in heaven, as the Greens want to phase out 20 coal-fired power plants and the FDP disagree. But there is a prototype coalition that has just begun in the northern state of Schleswig-HolsteinA Black-Yellow coalition of the CDU/CSU and FDP. Preferable for Mrs Merkel but the numbers in the Bundestag may not add up. A black-yellow coalition governed Germany from 2009 before the FDP's utter wipe-out in the 2013 electionAnother “Black-Red” alliance, of CDU/CSU and SPD, which governed Germany from 2005-2009 and then again from 2013-17. A realistic option in terms of numbers, but many in the SPD would be opposedRecent polls suggest the CDU/CSU has a double-digit lead over the SPD but there are millions of undecided voters - perhaps a third of the electorate.With the SPD still around 14 points behind Mrs Merkel's party in the polls, Mr Schulz is unlikely to attract all of them.The AfD, a nationalist, anti-Islam party, has so far held seats only in regional government but has grown in popularity during a campaign focused on immigration issues.Mainstream parties have already ruled out talking to the AfD, which has been polling at between 11% and 13% and could emerge as Germany's third-strongest party.Another significant contender is Die Linke (the Left).Some 61.5 million people aged over 18 are eligible to vote in federal elections, which are held every four years.There are two separate votes - one for a candidate in their constituency, and one for their preferred political party.Source: BBC