Germany's Bundestag lower house of parliament could swell to a size never seen before after today's election, thanks to a complex voting system that combines directly elected MPs with proportional representation.

Post-war Germany's election system mixes the "winner-takes-all" approach of Britain and the United States with the proportional representation system that allows for more small parties.

When German voters enter the polling booth, they make two crosses on the ballot paper - one for a direct representative in their local district, the other for their preferred political party.

The first vote is meant to ensure that each of Germany's 299 districts is represented in the Bundestag, while the second vote determines the total proportion of seats each party will eventually have in parliament.

Ahead of election day, the parties write up "candidate lists" in each of the 16 states. The names at the top have the biggest chance of getting a seat.

The party with the most votes then gets to send the most lawmakers to the lower house.

Overhang seats

For example, if a party scores three direct seats through the first vote but is eligible for ten seats overall through the second vote, seven more names on the party's list are also given seats.

A complication arises when the direct and party votes are out of balance because voters "split" their ballot.

When a party earns more direct seats than it is entitled to through its share of the party vote, it is granted the extra seats anyway. These are called "overhang" seats.

As a result, the Bundestag can expand far beyond its minimum size of 598 seats. After the 2017 election, the chamber had 709 lawmakers - a figure which could even grow larger.

An even bigger parliament could emerge this year because large numbers of voters are expected to split their votes.

Most women, most parties

A total of 60.4 million people over the age of 18 are eligible to vote for the next government of the European Union's most populous nation and its biggest economy. Women voters outnumber men at 31.2 million and 29.2 million, respectively.

About 2.8 million are first-time voters today.

Voter participation four years ago stood at 76.2%, up nearly five points from 2013 and higher than in many other Western democracies.

This year, 33% of the candidates for the Bundestag, which ultimately elects the chancellor, are female - a post-war record.

A whopping 47 parties are fielding candidates - another first.

Five-percent hurdle

Parties which score below 5% of the second vote stay out of parliament altogether. This is meant to prevent excessive political fragmentation and stop potentially extremist parties.

The far-left Die Linke party is flirting with the 5% bar in the polls and its success or failure could be a key factor in post-election coalition arithmetic.

Once polling booths close at 6pm local time, the question will be whether any alliance of parties has an absolute majority to elect a chancellor - half of all the lower house seats plus one.

2017 results

Here is where things stood after the 2017 election:

Christian Democratic Union (CDU) / Christian Social Union (CSU): 32.9% - 246 seats

Social Democratic Party (SPD): 20.5% - 153 seats

Alternative for Germany (AfD): 12.6% - 94 seats

Free Democrats (FDP): 10.7% - 80 seats

Die Linke: 9.2% - 69 seats

Greens: 8.9% - 67 seats