William Tronzo,  The Cultures of his Kingdom: Roger II and the Capella Palatina in Palermo (Princeton: Princeton University Press, 1997) 152

"We can only assume that both craftsmen and designs were sought out from the indigenous cultures in which they were produced and imported into Sicily .  .  .  Roger's expenditures .  .  . represented a truly extraordinary effort.  .  .  . In the Regnum of Sicily in which many languages were yoked .  .  . only one primary medium, the visual, provided a single basis for all to understand."