There have been times in the history of architecture when style was inevitable. In the classic period of Greece or in the Gothic period of northern Europe no architect raised the question as to the style in which he should construct a building. That was decreed for him. And we shall perhaps not go astray if we suggest that the inevitableness of that decree was determined by two factors. One was the purpose to be served by the building, the other was the control over the materials. The one factor determined the contents, the other the form in which those contents were to be expressed. The contents depended on the social and spiritual ideals of the time. The form depended on the nature of the building material and on the mechanical ability to use it.