Byzantium is an elusive phenomenon because so many of its constituent parts altered in place and over time. The overarching façade of the imperial order remained, with certain fixed points: religious doctrine, use of the Greek language, and the City of Constantinople itself. But many other elements were mutable – from court fashions, administrative methods and commercial undertakings, to forms of warfare or territorial possessions. Byzantium’s distinctive qualities lie in this interplay between the fixed and the changeable, the expendable and the non-negotiable, ensuring its endurance across a millennium or so, longevity which only the Chinese and Japanese empires can unequivocally be said to have surpassed.