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The Queen's peace (or, during the reign of a male monarch, King's peace) is the term used in the Commonwealth realms to describe the protection the monarch, in right of each state, provides to his or her subjects. In republics with common law traditions, the same is often referred to as the peace [and dignity] of the State.
Duty of the Crown to maintain peace 
Maintenance of the Queen's peace is one of the duties of the Crown, carried out via the Royal Prerogative. Though this power remains the Crown's, through convention it is exercised by the Queen-in-Council; that is, the executive, or, the sovereign acting on the advice of her ministers of the Crown.
The Crown can be held responsible should it fail in upholding its duty to maintain peace; this was the justification for the Riot Act and subsequent legislation throughout the British Empire. Where civil authorities had declared the Queen's peace as breached (i.e. there was a state of riot), there was a change in the rules: the authorities (whether police, army, or militia providing military aid to the civil power) could shoot and kill the leaders of the riot, and generally take severe action against anyone who was rioting. The counterbalance was that the Crown was responsible for damage caused by the riot, having failed in its duty to preserve the peace. Into the present day, the criminal offence of rioting can only be prosecuted as such with the consent of the Attorney-General (the Queen's legal officer). If disorder does occur but is not prosecuted as rioting, it is officially called a civil disturbance, as deeming it a riot transfers the liability of insurers for any damages or injury occurring from such an event to the local police, which, as they are officers of the Crown, makes the Crown liable to pay.
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Officers of the Queen's peace have the right to detain a person who is creating a breach of the peace. This is not a criminal or civil offence; it exists as a legal oddity created by the Royal Prerogative. Persons so detained must be taken before a magistrate (a Justice of the Peace), who will bind them over to keep the peace, whereafter the person may not disturb the peace again for the appointed time, under threat of imprisonment. The police will frequently use this power to break up difficult situations or minor fights; often, a perpetrator will be detained only briefly, until the officers are satisfied that the fight is over. Alternatively, if alcohol is present, for instance, the offender can be held until sober enough to face the magistrate. Because a breach of the Queen's peace is not a criminal offence, people found to have broken it will not have the charge marked on their criminal record.
Murder remains a common law offence, defined as "Where a person of sound mind and discretion unlawfully kills any reasonable creature in being and under the Queen’s peace with intent to kill or cause grievous bodily harm." However, the Queen's peace excludes killing of the enemy during a war.
Justice of the Peace 
Historically, and in particular before the founding of the police and the modern legal system, the concept of the Queen's peace was much more important. Knights of the Peace were appointed in each shire, and it was their duty to maintain the Queen's peace. These Knights of the Peace later became known as Justices of the Peace, or JPs, and subsequently as magistrates. In the United Kingdom, paid magistrates are now called District Judges, and are drawn from the ranks of local solicitors and barristers. Unpaid magistrates are volunteers from the community – the requirements are that they must be of good character and local residence.
Influence on other legal systems 
In the United States, arrest warrants and charging documents, such as indictments, are often constitutionally required to make reference to an offense having occurred "against the peace and dignity of" the respective state or commonwealth.
In the county palatine areas of the United Kingdom – the Duchies of Lancaster and Cornwall, and the County Palatine of Durham – offences such as murder were deemed to be against the respective bishop's or duke's peace (the Duke of Lancaster being merged with the Crown, but nevertheless a separate office, and the Duke of Cornwall being the Heir to the Throne). This, however, was altered in 1536.
See also