As negotiations between the Conservatives and the Liberal Democrats continue, speculation remains rife that voting reform could prove a sticking point between the two parties.

Lib Dem leader Nick Clegg wants to see a fundamental overhaul of the way people vote, describing the current first-past-the-post (FPTP) system as "outdated" and "broken".

In Thursday's election, the Lib Dems won 23% of the votes, but ended up with less than nine per cent of the seats in Parliament. Mr Clegg now wants proportional representation (PR) to replace FPTP to give smaller parties wider representation in the Commons.

But what are the options? The Liberal Democrats first choice would be to adopt the Single Transferable Vote (STV) system as it is the purest form of proportional representation. STV is also backed by the Electoral Reform Society, however, there are other PR systems which are seen as returning equally fair results.

Here is STV's guide to STV, and the other PR contenders.

What is Single Transferable Vote (STV)?

The Single Transferable Vote system is a form of proportional representation which maintains local links between politicians and voters. Candidates stand for multi-member districts rather than single-member constituencies, with voters marking the candidates in order of preference.

How does STV work?

Voters would mark candidates with numbers rather than a cross, numbering the candidates in order of preference. Candidates would then need to reach a known "quota" or number of votes to be elected. Second votes are distributed, or transferred, from those who are comfortably elected or who have so few votes they can be eliminated from the running, until the required number of politicians are elected. It is currently used for Scottish local council elections.

What are the advantages?

The system is seen as fairer because it results in far fewer votes being wasted than in FPTP. The multi-member aspect also encourages local competition between candidates, even those standing for the same party.

STV allows voters to vote for candidates rather than parties, and can allow voters to choose one candidate over another if they are standing for the same party. The results returned are broadly proportional,and the transfer aspect means more votes are influential. The system also eliminates the need for tactical voting.

What are the disadvantages?

STV will often not return an overall majority for one party, meaning negotiations between parties and coalition government would become far more common. Critics of the system also say it breaks down the local constituency link, with much larger districts.

The process of transferring votes can also be time-consuming and election results would be announced much later than under the current system.

What are the PR alternatives to STV?

There are a number of other systems which return more proportional results to FPTP and which could be preferable to the Conservatives.

Alternative Vote (AV)

Constituencies remain the same, but voters select candidates in order of preference. If a candidate gains more than 50% of the vote, they are elected. If no-one achieves a majority, second preference votes are redistributed until a candidate reaches 50%. The system is preferred by some because it retains the local constituency link between MPs and voters. It also means that everyone elected will have achieved support from the majority of voters in their constituency, even if not on first-preference votes.

Alternative Vote Plus (AV Plus)

This would see the majority of MPs elected using the AV system, with 150 additional seats allocated on a top-up basis. As well as voting for a constituency MP, voters would get a second vote to cast at county level or equivalent. The system helps avoid a situation where, like on Thursday, a party can end up with a far lower proportion of seats than their percentage share of the vote. AV Plus maintains the local constituency link and returns a proportional result, however, it has never been tested and critics argue it could result in confusion between the two different classes of MP.

Additional Member System

The hybrid system used in the Scottish Parliament elections gives everyone two votes - one for a candidate and one for a party. Local MPs are elected using the FPTP system, then the party votes are used to allocate MPs from a party list to ensure the make-up of parliament more evenly matches the election result. The system is seen as easy to understand, meaning little change for voters, but still returns a reasonably proportional result. However, it does mean that some MPs are not directly elected, as parties make up their candidate lists themselves, and the two classes of member can prove confusing. List MPs would also have less of a local link to voters than their constituency colleagues.