Identifying styles by common characteristics is an important part of looking at buildings, but there is much more to styles than this. In his book The Classical Language of Architecture, John Summerson said that it was a mistake to try to define classicism. Classicism was not just about the use of particular forms, it was also about the classical spirit and philosophy which informed their use. Consequently the term had different meanings in different contexts. Here we explore what we mean by styles and examine some of the ways in which architectural history is divided into periods. The concept of a period is drawn from different disciplines and this influences our perception of our subject.