Suggestions range from 1926 to 1971 based on the events outlined below:  Because of this separation, many independent Christian churches and churches of Christ are not only non-denominational, they can be anti-denominational, avoiding even the appearance or language associated with denominationalism holding true to their Restoration roots. One of the issues leading to the 1906 separation was the question of organizational structures above the level of the local congregation. Since then, Churches of Christ have maintained an ongoing commitment to church governance that is congregational only, rather than denominational. Churches of Christ purposefully have no central headquarters, councils, or other organizational structure above the local church level. Rather, the independent congregations are a network with each congregation participating at its own discretion in various means of service and fellowship with other congregations (see Sponsoring church (Churches of Christ)). Churches of Christ are linked by their shared commitment to restoration principles. Since Churches of Christ are autonomous and purposefully do not maintain an ecclesiastical hierarchy or doctrinal council, it is not unusual to find variations from congregation to congregation. There are many notable consistencies, however; for example, very few Church of Christ buildings display a cross, a practice common in other Christian churches. The approach taken to restoring the New Testament church has focused on "methods and procedures" such as church organization, the form of worship, and how the church should function. As a result, most divisions among Churches of Christ have been the result of "methodological" disputes. These are meaningful to members of this movement because of the seriousness with which they take the goal of "restoring the form and structure of the primitive church." Three quarters of the congregations and 87% of the membership are described by "The Encyclopedia of the Stone-Campbell Movement" as "mainstream", sharing a consensus on practice and theology. The remaining congregations may be grouped into four categories which generally differ from the mainstream consensus in specific practices, rather than in theological perspectives, and tend to have smaller congregations on average. The largest of these four categories is the "non-institutional" churches of Christ. This group is notable for opposing congregational support of institutions such as orphans homes and Bible colleges. Approximately 2,055 congregations fall in this category. The remaining three groups, whose congregations are generally considerably smaller than those of the mainstream or "non-institutional" groups, also oppose institutional support but differ from the "non-institutional" group by other beliefs and practices:   While there are no official membership statistics for the Churches of Christ, growth appears to have been relatively steady through the 20th century.