In England, as in the United States, most prisoners are held in prisons constructed more than a century ago. Prisons are classified administratively as local or central prisons. Local prisons serve a variety of purposes-holding prisoners awaiting trial or sentencing and prisoners serving shorter sentences (up to about 18 months). There the worst overcrowding occurs. Prisoners serving longer sentences are detained in central prisons, dealing exclusively with similar cases. For security, prisoners are classified into four categories, from A (prisoners likely to attempt escape, and constituting, if successful, a significant danger to the public) to D (prisoners who can be trusted to work in conditions of minimal security). Central prisons cover a range from maximum-security institutions to medium-security prisons, where the degree of security is less intense; and to open prisons, where physical security is minimal and there is normally no obstacle to a prisoner's absconding. In some European countries a further category of institution is available to accommodate prisoners who are allowed to serve their sentences intermittently, usually over a series of weekends. Younger offenders in England (in the age group 15-21) were until recently held in Borstal institutions, named after the village in Kent where the first one was operated. For many years these institutions were admired as an example of practical rehabilitation through training, but declining enthusiasm for this concept, and disillusionment with its effectiveness, led to its replacement with that of "youth custody." Another feature of the English prison system is the detention centre. These institutions for young males serving sentences that must not exceed four months are based on the principle of vigorous discipline and physical activity, popularly known as the "short sharp shock"; research has failed to show, however, that it is an effective deterrent to further crime.
Prisons have been described as total institutions, in which every aspect of life is subject to control. In addition to daily routines such as mealtimes, times of rising and retiring, and bathing, many other aspects of the prisoner's life are subject to control. In part this control forms the deprivation of freedom that is the essence of imprisonment, and in part it is a necessary adjunct as a means of maintaining security, controlling the introduction of weapons or contraband substances, and preventing escapes. Most prisons limit the number of visits that a prisoner may receive from his family or friends. In England the Prison Rules allow a convicted prisoner one visit every four weeks, although the prison governor may increase or limit visits at his discretion. Only relatives and friends of the prisoner may visit him, although adequate facilities must be available for visits by legal advisers if the prisoner is engaged in any litigation (for instance, divorce proceedings). Visits normally take place within the sight of an officer, and in some cases within his hearing. In many prisons, visits are conducted with the prisoner sitting on one side of a table and his visitor on the other, with a wire mesh partition between them; the visitor may be searched for contraband. In other prisons the conditions for visiting may be less restrictive-the visitor and the prisoner may be allowed to meet in a room without any physical barrier but still