The whip is a political party official who ensures that elected Members of Parliament toe the party line during House debates and votes.

Lifting the whip in a parliamentary vote allows MPs to vote freely instead of along party lines.

Technically, it is not necessary to lift the party whip for MPs to speak freely in House debates, but doing so sends a signal that they are free to debate as well as vote.

This system is used by parties in parliamentary democracies, such as the United Kingdom and Australia. The Workers' Party, the only opposition party in Singapore with MPs, has the same system.

In the UK, parties have elaborate whips - from one to three lines - to indicate how important the issue is and how seriously dissent will be dealt with. Defying a three-line whip can lead to the whip being withdrawn from an MP, which means the member is effectively expelled from the party and has to sit as an independent until the whip is restored.

Here are some key issues where the People's Action Party (PAP) lifted the party whip in Parliament in the past.

1. Abortion Bill, 1969

The Abortion Bill was introduced in Parliament on December 1968 to decriminalise abortion, which was then a crime under the Penal Code.

The Bill was debated over a marathon three-day session from April 8 to 10, 1969. It was sent to a select committee to examine and gather public feedback.

The committee received 33 written submissions and heard five oral representations.

The Bill went back to the House for a vote on Dec 29, 1969. It passed with 32 "yes" against 10 "no" votes. One MP abstained, while 15 were absent from the vote.

Among those who voted against the Bill were MPs Ho Kah Leong, Ho See Beng and Ng Kah Ting.

Then Prime Minister Lee Kuan Yew and Minister for Science and Technology Toh Chin Chye were absent for the vote, even though they attended the Parliament sitting.

Education policies, 1979, 2002 and 2004

The whip was lifted thrice when major education policies were debated in the House.

Lifting the whip in a parliamentary vote allows MPs to vote freely instead of along party lines.

Between March 27 and 30, 1979, Parliament debated the paper Revised Structure Of Education tabled by then Deputy Prime Minister and Education Minister Goh Keng Swee. The key proposal of the paper was to introduce streaming and bilingualism in schools.

Twenty seven MPs took part in the debate, apart from the Education Minister.

After the four-day debate, the House unanimously passed the motion to support the paper.

The second time the whip was lifted for education policies was between Nov 25 and 27, 2002, when Parliament debated the Junior College and Upper Secondary School Education Review Committee report. Among the proposed changes was the through-train system where students can skip the GCE O levels.

Elitism was the main issue that emerged in the debate, with some MPs worried that only top students would benefit from the changes, while others said that both strong and weak students would be looked after.

The issue causing the whip to be lifted the most times was the Nominated MP scheme.

In November 2004, the whip was lifted for the debate on the White Paper on Chinese Language Curriculum and Pedagogy Review.

The report proposed reforms to Chinese-language learning in schools.

After a three-day debate from Nov 24 to 26, PAP MPs unanimously supported the paper; Mr Chiam See Tong, who was then Singapore People's Party chief, said no; while Workers' Party secretary-general Low Thia Khiang abstained.

2. Human Organ Transplant (Amendment) Bill, 2004

On Nov 10, 2003, Acting Health Minister Khaw Boon Wan proposed changes to expand the Human Organ Transplant Act to cover the harvesting of kidneys, liver, heart and corneas from people who died.

It had allowed only kidneys to be harvested from accident victims.

After a two-day debate in Parliament on Jan 5 and 6, the proposed changes were passed without any dissent, even from opposition members Mr Chiam and Mr Low.

3. Nominated MP scheme, 1992, 1997, 2002 and 2006

The issue causing the whip to be lifted the most times was the Nominated MP scheme.

It was first introduced in Parliament on Oct 6, 1989, through proposed changes to the Constitution.

When the scheme was debated a month later, in November, some MPs called for the whip to be lifted, but then First Deputy Prime Minister and Minister for Defence Goh Chok Tong said no.

But Mr Goh made a concession: Future terms of government could decide, after the general elections, whether to continue to debate and decide whether to have the scheme.

He promised that the whip would be lifted in these debates.

The scheme was introduced in November 1990 when cardiologist Maurice Choo and company executive Leong Chee Whye were appointed the first two NMPs.

Mr Goh's concession led to the whip being lifted in May 1992, June 1997, April 2002 and November 2006 when the new terms of Parliament voted to continue the NMP scheme. Mr Tan Cheng Bock, then MP for Ayer Rajah, spoke up strongly against the scheme and voted against it in June 1997 and April 2002.

On April 26, 2010, Parliament voted to make the NMP scheme a permanent one by embedding it in the Constitution.

This meant that Parliament no longer has to vote on whether it wants NMPs for each new term.

The whip was not lifted for the vote.

4. Maintenance of Parents (Amendment) Bill, 2010

The debate on the changes to the Maintenance of Parents Act on Nov 23, 2010 was when the whip was last lifted in Parliament.

The proposed changes enabled parents to sue their children in court for financial support.

The changes were not proposed by the Government but by Marine Parade GRC MP Seah Kian Peng, who was then chairing the Government Parliamentary Committee for Community Development, Youth and Sports.

Eighteen spoke at the debate, excluding Mr Seah.

The Bill was passed.