Speaker Denison's rule is a constitutional convention established by the 19th century Speaker of the British House of Commons John Evelyn Denison regarding how the Speaker decides on his casting vote in the event of a tie.
The principle is to always vote in favour of further debate, or, where no further debate is possible, to vote in favour of the status quo. For example, the Speaker will vote:
The thinking behind the rule is that change should only occur if an actual majority vote is in favour of change.
Speaker Denison's rule is now a guiding principle in many other bodies that have neutral chairpersons.