When Princess Diana appeared at the 1987 Cannes Film Festival in a flowing powder-blue gown, most saw only a fashion moment. But the dress, designed by Catherine Walker, carried a quiet tribute to another royal icon—Princess Grace of Monaco.

Diana and Grace had met in 1981, shortly after Diana’s engagement. Overwhelmed, Diana broke down in tears, and it was Grace who comforted her, offering advice only another royal outsider could. Grace tragically died in a car crash in 1982—a fate that would later mirror Diana’s. The gown’s icy hue and silhouette echoed the dress Grace Kelly wore in To Catch a Thief, filmed in the French Riviera where she met Prince Rainier.