This chapter is concerned with the eastern arm of the cathedral, as extended and remodelled in the second half of the 13th century. It examines the evidence for the style of the building in greater detail than has been attempted hitherto, and sets its features in the context of English ecclesiastical architecture during the experimental period when Early English and French Rayonnant ideas intermingled at the beginning of the Decorated style. The chapter demonstrates that the execution of the work was a slow process, so that it is unlikely that it was all designed at its inception. The irony of St Paul’s is that the importance of its workshop in these years was not matched by speed of completion of the work. By the time that the eastern arm was finished and ready to view, its decorative vocabulary had been superseded by further developments, especially in vault patterns and window tracery, in workshops in the west and in East Anglia.