Speaker Denison's rule is a constitutional convention established by John Evelyn Denison, who was Speaker of the British House of Commons from 1857 to 1872, regarding how the Speaker decides on their casting vote in the event of a tie in the number of votes cast in a division.
In 1867, when a tie arose on a motion on Fellowships at Trinity College, Dublin, Denison gave his casting vote against the motion, declaring that any decision must be approved by the majority. The rule as subsequently adopted is that the Speaker, in any division upon a bill, should vote to leave a bill in its existing form.
The principle is always to vote in favour of further debate, or, where it has been previously decided to have no further debate or in some specific instances, to vote in favour of the status quo. Thus, the Speaker will vote:
The thinking behind the rule is that change should only occur if an actual majority vote is in favour of the change.
Speaker Denison's rule is now a guiding principle in many other bodies that have neutral chairpersons.
In the case of a Committee of the Whole House, the presiding officer is the Chairman of Ways and Means or a Deputy Chairman. In other cases of plenary session, the presiding officer is the Speaker or a Deputy Speaker. Votes of smaller Commons committees are not listed.
The Speaker of the House of Commons of Canada in 2005, Peter Milliken, broke a tie on a confidence motion to pass the 2005 Canadian federal budget; this is the first and only time a tie on a confidence motion has occurred in Canada. Milliken cited parliamentary precedent while explaining his decision.