The aer is the large rectangular cloth that goes over Chalice and Diskos at the end of the proskomedia, and again at the end of the Great Entrance; it’s also what Father is in there flapping around during the Nicene Creed; but what is it?! The aer (literally “air”) was originally used to prevent anything from falling into the Chalice prior to consecration, but also symbolizes both the swaddling clothes of the infant Christ as well as His burial shroud.
Like the antimens (more on that in a future writing), it is folded 3x3, so that the creases form a cross when it’s laid out flat. There is a cross embroidered on the center of the aer so that when it is folded, the cross is visible - this will be placed on the diskos along with the star, spoon, and one of the veils, and placed on the proskomedia (preparation) table after Holy Communion.
The aer is utilized in the priestly ordination, and at the death of a priest (or bishop) when the body is vested for burial, his face will be covered with an aer to show his closeness to the Sacred Mysteries. See if you can spot this important piece of Liturgical gear next time you’re at the parish!