Plain people are Christian groups characterized by separation from the world and by simple living, including plain dressing. Many Plain people have an Anabaptist background. These denominations are of German, Swiss German or Dutch ancestry. Conservative Friends are traditional Quakers who are also considered plain people; they come from a variety of different ethnic backgrounds.
The Mennonite movement was a reform movement of Anabaptist origins begun by Swiss Brethren and soon thereafter finding greater cohesion based on the teachings of Menno Simons 1496–1561, and the 1632 Dordrecht Confession of Faith. The Amish movement was a reform movement within the Mennonite movement, based on the teachings of Jacob Ammann, who perceived a lack of discipline within the Mennonites movement by those trying to avoid prosecution. Ammann argued that Romans 12:2 prohibited that.
William Penn, having experienced religious persecution as a Quaker, offered asylum to others who were suffering religious persecution, an offer that many followers of Jacob Ammann accepted, starting with the Detweiler and Sieber families, who settled in Berks County, Pennsylvania, in 1736. Many of them settled near Lancaster, Pennsylvania, which offered some of the most productive non-irrigated farmland in the world. By 1770, the Amish migration had largely ceased.
Notable Plain groups in the Anabaptist tradition are Old Order Amish, New Order Amish, Kauffman Amish Mennonites, Beachy Amish Mennonites, Para-Amish ( Believers in Christ, Vernon Community, Caneyville Christian Community), Old Order Mennonites, Conservative Mennonites, Conservative Mennonite Conference, Reformed Mennonites, Orthodox Mennonites, Old Colony Mennonites, Holdeman Mennonites, Hutterites,  Bruderhof,  Old German Baptist Brethren, and Old Order River Brethren. The Cooperites (Gloriavale Christian Community) also wear plain dress. 
Early Methodists wore plain dress, with clergy condemning "high headdresses, ruffles, laces, gold, and 'costly apparel' in general".  In his sermon On Dress, John Wesley, the founder of the Methodist movement, expressed his desire for Methodists to wear plain clothing in the manner practiced by Quakers: "Let me see, before I die, a Methodist congregation, full as plain dressed as a Quaker congregation."  Peter Cartwright, a Methodist revivalist, noted the gradual decline of wearing plain dress among Methodists;  today, members of denominations in the conservative holiness movement, such as the Allegheny Wesleyan Methodist Connection and Evangelical Wesleyan Church, continue to dress plainly,   sometimes including abstention of wearing of jewelry, such as wedding rings. 
Customs of Plain people include:
- Plain clothes, usually in solid, normally dark colors.
- Plain church buildings, or no church buildings whatsoever.
- A utilitarian view of technology, similar to the precautionary principle of technology in that unknowns should be avoided, but the emphasis was on the results in the eyes of God. If they were unsure of how God would look upon a technology, the leaders of the church would determine whether it was to be avoided or not.  The degree to which this principle was supported varied among the congregations, but in general, the Amish people believed that the Mennonites had not done enough to separate themselves from the rest of the world.
Anabaptist Plain groups typically have a bishop presiding over one congregation ( Amish) or over a district (group of congregations) ( Old Order Mennonites). Mennonites mostly meet in church buildings, but most Amish meet in members' homes. Services among Amish and Plain Mennonites are mostly held in Pennsylvania German, a language closely related to Palatinate German, with extra vocabulary. Bishops are commonly chosen by lot as a reflection of God's will. While the Bishop tends to be influential, he tends to rule by building consensus rather than by issuing edicts.
Most Anabaptist Plain groups have an Ordnung that among other things regulates clothing. The Ordnung is a largely unwritten code of behavior, covering such items as clothing, vehicles, and the use of technology. The Ordnung varies slightly from congregation to congregation, though is in essence the same. Violations are not considered sins, although wilfulness is considered to be a serious violation of the faith. The congregation can change the Ordnung if there is a majority to do so. Exemptions to the Ordnung can be provided. In one instance, one farmer was granted permission to buy a modern tractor since he had arthritis and no children to help him harness horses.
The Old Order Amish are among the fastest-growing populations in the world. They have low infant mortality rates. The average Amish woman can expect to have at least seven live births. [ dubious ] Other Plain sects with the same or similar doctrines can be expected to have similarly explosive growth.
Despite this, the Pennsylvania Dutch, which includes Amish, Old Order Mennonite, and Conservative Mennonites are expected to become a smaller percentage of the population as the sects respond to high prices of farmland by spreading out all over the United States and internationally, and the English (the Amish exonym for non-Amish persons regardless of ancestry) population spreads out from Philadelphia into suburban and rural areas. Donald Kraybill believes there are Plain sect communities in 47 states.
Among people at least five years old living in Lancaster County in 2000,
Most Anabaptist Plain sects do not admit children to their church, and impose no sanctions on those who do not join, but shun those who fall away from the church once becoming a member. Among some groups of Old Order Amish, teenagers who are not yet baptized are not bound by the rules and go through a period of rumspringa, often with a certain amount of misbehavior that would not otherwise be tolerated.
The Pennsylvania Dutch generally do not proselytize and discourage intermarriage. Because of close consanguinity, certain genetic problems occur more frequently. Dr. D. Holmes Morton has established the Clinic for Special Children to study and treat families with these problems. 
The Plain sects typically prohibit insurance, and they assist each other charitably in case of sickness, accident, or property damage. Internal Revenue Service Form 4029  allows one to claim exemption to Social Security taxes under certain very restrictive conditions, and members of the Plain groups neither pay these taxes nor receive death, disability, or retirement benefits from social security.
- Christian headcovering
- Fancy Dutch
- Haredi Judaism
- Peace Churches
- Testimony of Simplicity
- Tolstoyan movement
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Should we insist on plain and modest dress? Certainly. We should not on any account spend what the Lord has put into our hands as stewards, to be used for His glory, in expensive wearing apparel, when thousands are suffering for food and raiment, and millions are perishing for the Word of life. Let the dress of every member of every Allegheny Wesleyan Methodist Church be plain and modest. Let the strictest carefulness and economy be used in these respects.
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