In law, a quorum is the minimum number of members of a deliberative body necessary to conduct the business of that group. By default, this is a majority of the people expected to be there, although many bodies may have a much lower quorum. For instance, the House of Lords can decide on procedural issues if a mere three members are present. By contrast, according to Article One of the United States Constitution, the United States House of Representatives and the United States Senate each have a quorum of a majority of their respective members. The Senate has the additional requirement in Rule VI of its standing rules of a "majority of the members duly chosen and sworn."
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When quorum is not met, a legislative body cannot hold a vote, and cannot change the status quo. Therefore, voters who are in favor of the status quo are able to use an obstructive strategy called quorum-busting. If a significant number of voters choose not to be present for the vote, the vote will fail due to lack of quorum, and the status quo will remain.
A prominent example of quorum-busting occurred in 2003, when the Texas House of Representatives was going to vote on a gerrymandering bill that would have favored the Republicans in the state. The House Democrats, certain of defeat if a quorum was present, chose not to be present in the House that day, but instead took a bus to Oklahoma, preventing the bill from passing due to a lack of a quorum. Legislative bodies often have rules to discourage quorum-busting. In many U.S. legislative bodies, such as the Senate, absent members can be arrested and brought to the floor. This was the reason that the Texas Democrats fled to Oklahoma, which is outside of the jurisdiction of Texas law.
Refusal to vote
The technique of the Disappearing quorum (refusing to vote although physically present on the floor), was used by the minority to block votes in the US House of Representatives until 1890.
Quorum in online communities
When votes are held in large online communities, where it may never be the case that a majority of the members are "present", the effect of quorum is different. Being absent from the vote no longer requires particular effort, but is the default case: voters are usually assumed to be absent unless they cast a vote. Online communities therefore tend to have quorums that are much less than a majority of the members.
In such votes, a non-monotonic aspect can be introduced: a voter can inadvertently swing a vote from failing to passing by voting "no", if a majority has voted "yes" and that "no" vote is the one that causes quorum to be met. With no penalty for being absent, voters are faced with a strategic choice between voting "no" and not voting.
The Debian project has addressed this issue in its voting mechanisms with the idea of per-option quorum. A quorum is not set on the total number of votes, but on the number of votes a particular option (besides the status quo) must receive before it is considered. For example, in a yes/no vote, the quorum may say that at least 40 "yes" votes are required, along with "yes" having a majority of votes, for the vote to pass.