Changes to social structure, primarily the reduction in the service class, led to many changes in the house plan during the early twentieth century. Several scholars have documented this evolution in the last century. In North America, the house evolved from a single room or hall into the complex series of rooms of today. The hall expanded to include a parlor and then to include a central hall. Double pile plans replaced the single pile plan and house plans were at their most complex during the end of the nineteenth and beginning of the twentieth centuries. Social reformers sought a return to simplicity as evident in the Colonial house in response to the excess of the Victorian era.