korsflagg and courtesy ensign

First prescribed as the proper and accepted way to identify Danish merchant vessels in regulations published on this day in 1748, specifying the colours of the flag (Dannebrog), shifting the intersection to the hoist (left) side and making the outer fields 6/4 the length of the inner ones, the distinctive Nordic Cross banner has since been adopted by Scandinavian and adjacent countries and territories.
One notable exception, though the design references the idea, is Greenland once granted home rule in 1985. Although the sideways cross is associated with Philip, the Apostle of the Greeks, who is venerated on 3/11 May (see also—coincidentally both Apostles Barnabas and Bartholomew are fêted on 11 June) dragging it to his own execution though by some accounts spared by the crowd by dint of his eloquent sermon, vexillogists employ the term Nordic cross for this and inspired conventions.