This portrait of Juan Carlos I of Spain shows how a dress shirt should peek out from under a jacket. There's half an inch of linen above the jacket collar at the rear, and half an inch at the ends of the sleeves (the photo was shot upwards, so the viewer sees a bit of cufflink that would be out of sight from a normal perspective).
Like his relative the Prince of Wales, Juan Carlos is usually seen in well cut double breasteds. The DB has a bit of the flavor of a military uniform, and evokes more of an aristocratic heritage than a single breasted coat.
Fit for a king.