Cross the floor

To cross the floor is an expression in British parliamentary life, in full to cross the floor of the House [of Commons]. It means 'to change sides', and specifically 'to transfer one allegiance from one political party to another, after having been elected in the interests of the first'. It is quite rare in British political life, but in most parliaments of the last hundred years there have been one or a few examples. Wikipedia has a list at [[1]].

Members of Parliament sit in parties. Those who are in the party which forms the government sit in the benches on on one side of the Chamber (on the right hand of the Speaker); members of other parties sit opposite them.

In the House of Lords, there is a third set of benches at right angles to the other two, facing the Lord Speaker, allowing a category of 'cross-benchers': those affiliated to the parties of neither the Government nor the official Opposition.