Net-aPorter described this dress:
"The claret, cobalt-blue and cream floral print on Erdem's 'Alouette' gown is inspired by 1960s Japanese graphics. Flawlessly cut from lustrous silk-gazar, this elegant piece is fitted at the bodice and flares below the waist into a voluminous peplum skirt. Show off the low V-back with an updo."
Kate did not wear an updo, but we still got a sense of the back. The dress is 100% silk and features a bold "ruffle" that split fashion fans, but certainly made a splash.