The image of the leaders of the SNP, the Green Party and Plaid Cymru hugging went viral after the BBC debate on Thursday. Commentators have highlighted the friendship between the "progressive alliance", but what do the three parties have in common? And what do they disagree on?

Women

It's stating the obvious to say all the parties have female leaders. But the Green's Natalie Bennett, Plaid Cymru's Leanne Wood and Nicola Sturgeon of the SNP stand out for their gender in the male-dominated world of politics.

There have been a total of 370 women MPs since 1918 at Westminster, for example. There were more men (502) elected to the House of Commons in the 2010 general election.

Trident

Image copyright PA Image caption The UK's nuclear weapons system has been based at HM Naval Base Clyde since the 1960s

The SNP, Green Party and Plaid Cymru all oppose the renewal of the UK's nuclear missile system - currently made up of four Vanguard-class submarines which carry Trident strategic missiles. The system is based on the Clyde, on Scotland's west coast. The SNP opposes nuclear weapons and wants Trident removed from Scotland. Plaid Cymru's manifesto calls the replacement of Trident "wasteful and unnecessary". The Green Party wants to decommission all existing nuclear forces and facilities.

Policy guide: Foreign and defence This election issue includes foreign policy and the role of UK’s defence forces at home and abroad.

Anti-austerity

Image caption The three parties want an end to austerity measures

The Green Party, Plaid Cymru and the SNP all want to see austerity scaled back. In its manifesto, Plaid Cymru says it does not support the continuation of "austerity" policies. A key part of the Green Party's manifesto is to end austerity and "transform" the UK economy in order to "close the gap between the rich and the poor". The SNP says it supports an alternative to austerity with "modest" spending increases.

Policy guide: Economy This issue includes the wider economy and deficit reduction but also employment and the role of business.

EU

Image copyright Getty Images Image caption The parties back EU membership

The three parties all back EU membership. Plaid Cymru says it will campaign to stay in EU in event of a referendum, while Ms Sturgeon has said being taken out of the EU against Scotland's will would be "democratically indefensible". The Green Party stands for staying in a reformed Europe, but Ms Bennett has said it wants to hold a referendum because the party "believes in democracy and self-determination".

Policy guide: EU This election issue includes the UK’s membership of the European Union and its negotiating position.

Oppose NHS privatisation

Image copyright Getty Images Image caption All the parties want to protect health budget

Health and care is a devolved power in Scotland and Wales, but the three parties all pledge to end what they call the "creeping privatisation" of the NHS.

Policy guide: Health and care This issue includes NHS funding, GP access and social care, particularly of older people.

Young voters

Image copyright David Cheskin Image caption More than 100,000 teenagers registered to vote in the Scottish referendum

The SNP, Green Party and Plaid Cymru all want to lower the voting age to 16. It comes after Scotland allowed 16 and 17-year-olds to vote in the Scottish referendum.

Policy guide: Constitution This election issue includes devolution and reform of the Westminster political process.

But there are also plenty of differences...

The SNP and Plaid Cymru only put up candidates in Scotland and Wales. The SNP is contesting all 59 seats in Scotland, and Plaid Cymru all 40 in Wales.

Ms Bennett is the leader of the Green Party of England and Wales, but there is also a Green Party in Northern Ireland and Scottish Green Party. Collectively, they are standing in 571 of the 650 seats.

The two nationalist parties also have their own agendas. The SNP wants Scotland to be independent (the Scottish Green Party was in favour of an independent Scotland). Plaid Cymru wants Wales to get the same powers and funding as Scotland, and harbours longer-term independence aspirations.

The Green Party wants further devolution within the UK. However, it ranks things like ensuring global temperatures do not rise by more than 2C and renewable electricity generation among its priorities.

In terms of immigration, the Greens' main pledges include removing restrictions on foreign students, abolishing some family migration rules, and giving asylum seekers more rights.

Plaid Cymru wants to create a Welsh Migration Service and draw up a skills-shortage list to ensure migration meets Welsh needs.

The SNP wants powers over immigration to be devolved to Scotland in the way they are in Australia's states, with adjustment of the points-based system for skilled migrants who choose to move to Scotland. It also wants graduates from Scottish universities to be able to work in Scotland for an agreed period.

Read the BBC's full policy guide to political parties' positions on key issues, which is being updated as each manifesto is launched.

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