The election for London mayor and the Greater London Assembly will be decided by a new voting system.
BBC News Online guides you through the process.

The vote for mayor

Voters will have two ballot papers and will be able to cast four votes - two for the mayor and two for the GLA.

The system to elect the mayor is known as the Supplementary Vote (SV) system and introduces the concept of preferential voting into London politics. Here's an example of what the ballot paper will look like:

Voters will be asked to cast a vote for their favourite candidate in the left hand column, just as they do at the moment at the General Election.

They can then place a second vote in the next column for their second choice for mayor.

If a candidate wins more than half of the total first choice votes cast, they are elected mayor.

But if no candidate gets more than half the votes, all but the top two candidates are eliminated.

Second preference votes for the two remaining candidates on ballot papers of the eliminated candidates are then counted. The candidate with the most first choice and second choice votes is elected mayor.

The system means that voters should consider which candidates will reach the final run-off - and therefore which way to vote tactically if they want their vote to count.

The Assembly voting form

The Greater London Assembly comprises 25 members, 14 of which are elected directly from the London constituencies and the remaining 11 through the Additional Member System (AMS). Electors cast one vote for each part of the system.

Here's what the ballot paper, complete with imaginary parties, will look like:

The first vote goes to a preferred constituency candidate.

The AMS vote is cast for a voter's preferred party across London as a whole.

Additional members are drawn from a "party list" depending on how many, if any, additional seats that party wins.

The AMS system

London's AMS system uses the D'Hondt system to calculate additional seats.

The system, most well known for its use in the Northern Ireland Assembly, seeks to reflect the strength of a party's total support by taking into account its share of votes in relation to the number of seats already won.

For this example, we have created a fictitious assembly of 15 seats, contested by three parties.

Ten of these seats are won directly in the constituencies. The remaining five are allocated through AMS.

Here are the results after the constituency counts:

The London New Party, (LNP) and the London for Ever Party (LEP) dominate the assembly at the expense of the Turn Again London Party (TALP), which has not won enough support to gain a constituency.

However, here are the totals after AMS votes have been counted:

The results show that TALP has won the support of around 20% of the electorate across the whole of London.

If a party takes at least 5% of the share of votes, the D'Hondt system is used to ensure that the party has a representation in the assembly.

The total number of AMS votes for each party is divided by the number of seats won in the constituencies plus one.

The extra one is added to allow the formula to work for parties who have not won any seats at all.

So the LNP's vote of 70,107 is divided by six (five constituency seats plus one). TALP's vote is divided by one as it did not win any constituencies at all.

The party with the most votes at the end of each round of division takes an additional member seat until they are all allocated. Here's the result of round one:

TALP is the clear winner and takes the first AMS seat. Its divider now moves to two and the parties go into the next round of voting:

TALP wins again. It now has two seats to the five each of the other two parties.

Here's the next three rounds of voting. Each time a seat is won by any of the parties it is added into the calculation.

The highest polling party in the AMS section. the LNP, takes two more seats because the divider takes into account its higher share of the vote compared to the LEP.

The LEP does not take any seats at all because its share of the constituency seats in relation to its AMS vote show that it is already over-represented

The smallest of the three parties, TALP, takes one final seat. Here's the final results:

It shows that each party has won a number of seats roughly equal to its share of the AMS vote. TALP took 20% of the seats on around 19% of the vote. AMS has ironed out the disproportionate results of the constituency section.

Electoral experts say that with 11 seats up for grabs in the real AMS section of the London elections, smaller parties have a realistic chance of entering the Assembly.

But critics predict that the 5% qualification level for AMS entry may still be too high and could act against smaller parties rather than merely preventing extremist parties and candidates entering the Assembly.