The Eighth Amendment (Amendment VIII) to the United States Constitution is the part of the United States Bill of Rights which prohibits the federal government from imposing excessive bail, excessive fines or cruel and unusual punishments.
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Like many of the provisions in the Bill of Rights, the Eight Amendment explicitly forbade certain practices that England had inflicted upon the colonies (and other nations had inflicted upon their subjects). It was felt by the newly-independent states that, if they agreed to concede some of their sovereign power to a new central government, assurances were needed to prevent the central government from becoming too powerful or imposing such things upon their citizens in the future.
Specifically, the Eight Amendment addressed excessive bail and unreasonable fines, as well as cruel or unusual punishment, because such things had, in fact, been imposed by the former mother-country, and wanted to make clear that no future Federal government would attempt to do the same to them.