This is from Pope Pius II's History of Frederick III, as quoted by Jacob Grimm in an article documenting the history of marriage rites. It concerns Frederick's marriage to Eleonora of Portugal:
[jacentique sibi Fridericus III juissit] Leonoram in ulnas complexusque dari ac praesente rege (Alphonso), cunctis proceribus adstantibus, superduci culcitram.
My attempt: "Frederick, having lay down, ordered that Leonora be given into his embracing arms and, in the presence of the King and of the assembled nobles, carried over the mattress." But that makes no sense; how can she first be given into his arms and then carried over the mattress? Can anyone help? Thank you!