Cologne, Germany

THE impressive Islamic complex here, Germany's largest, boasts towering minarets and a soaring prayer hall. But what Turkish officials here seem most proud of are the hundreds of windows, which allow outsiders and Muslim worshippers to glimpse each other's worlds. The idea, they say, is transparency.

Yet it is what lies beneath the surface these days that concerns both Germans and Turks as a Turkish referendum starts on Sunday that could vastly expand the powers of its already authoritarian president, Recep Tayyip Erdogan, whose reach into Germany - both open and concealed - has become an increasing point of friction.