The term floor-crossing was first used to describe the process when Members of the British House of Commons crossed the floor to join the group of people (members of another political party) that was seated on the opposite site of the floor. Today, the process whereby one Member of Parliament (or Council) ultimately leaves his or her political party in order to join another party or become an independent candidate is often referred to as floor-crossing.  

It should be noted, however, that in some cases the term floor-crossing is used to denote the singular event of voting with another party on one particular and special question. This is the case in a few countries, where the Members of Parliament vote through their seating position and are allowed to change their seat for every vote, particularly in decisions without required party discipline.

Considering the first usage of floor-crossing (when Members of Parliament leave their political party), the question is what then happens to the seat? To whom does the seat belong? There are three main ways that legislation can deal with this:

The seat belongs to the political party

• If this is the case, the person who leaves (or is expelled from) his/her political party will lose their seat, and the party can decide whom they want to give it to. This strengthens the party organisation and keeps the political balance that voters decided on in the last election. This system is most common in countries with List PR electoral systems.

The seat belongs to the individual Member of Parliament, and he or she can keep the seat regardless of if they stay in the same political party

• This strengthens the role of the individual and also his or her links to the constituency. In this case, party discipline may be weakened, and some argue that individual Members of Parliament are less likely to seek consensus with party members and in the parliamentary group if they have the option to leave the party but to keep their seat at the same time. Floor-crossing is also seen by some as a justified way to respond to a changing political context and something that should be part of a dynamic and vivid party democracy.

The seat belongs neither to the party nor to the individual Member of Parliament.

• A by-election must be held to fill the seat. This system is used in countries with majority/plurality electoral systems. A by-election offers the opportunity to fill the vacant seat with a fresh candidate, and it allows the voters to express their will again. Voters may vote differently and thereby express discontent with the ruling parties.

N.B. The views of members of the ACE Practitioners' Network on this question can be read in the consolidated reply: Elected Member of Parliament not allowed to stay on as independent MP after leaving party: Examples?