Although the kings called Christian have *usually* named their eldest son Frederik, and vice versa, there is at least one instance where this was not the case, and where the regular alternation of names came about by chance rather than intentionally: Christian IV's eldest son was called Christian, and if that son had not predeceased his father by a year, Christian IV would have been succeeded by Christian V. Instead, the "regularity" was preserved as the younger son Frederik succeeded his father instead, becoming Frederik III.

When the childless Frederik VII died in 1863, the first king of the house of Glücksburg came to the throne. But as it happened, he was called Christian, so that Frederik VII was succeeded, quite "predictably", by Christian IX. However, when he was born, as the fourth son of a minor German duke, nobody could have known that he was destined for the Danish throne, and his parents could of course have chosen to give him a completely different name...