The tradition's institutional expression began to organize in 1908, when thirty two Christian communions met in Philadelphia to form the Federal Council of Churches of Christ in America, the first sustained cooperative body across what would become the mainline denominations. William R. Hutchison's 1989 book named the tradition's seven most institutionally dominant members the Seven Sisters of American Protestantism: the American Baptist Churches USA, the Christian Church (Disciples of Christ), the Episcopal Church, the Evangelical Lutheran Church in America, the Presbyterian Church (U.S.A.), the United Church of Christ, and the United Methodist Church.
This atlas currently holds six of the seven as full entities: the Evangelical Lutheran Church in America, the Episcopal Church, the Presbyterian Church (U.S.A.), the United Church of Christ, the United Methodist Church, and the Christian Church (Disciples of Christ), recorded here as Disciples of Christ, each with its own description, comparable facts and sourced edges into the Christian tradition. One is not yet an entity here: the American Baptist Churches USA, a loosely federated body whose individual congregations and regions retain wide autonomy over their own positions. Naming that gap honestly, rather than filing it under a denomination it is not, is what the family classification exists to make possible: six real member churches, sourced and comparable, standing under one honestly labeled family, with the one still missing named rather than silently absent.