Ad Orientem: When the priest conducts the Mass facing away from the people, looking at the wall behind the Altar.
Summorum Pontificum: Apostolic Letter written by Benedict 16 where he specified the circumstances in which priests of the Latin Church may celebrate Mass according to what he called the “Missal promulgated by Blessed John XXIII in 1962”, and administer most of the sacraments in the form used before the liturgical reforms that followed the Second Vatican Council.
Clarity. They’re facing east, as most churches in the shape of a cross are oriented toward the east. The priest is looking at Christ, not the wall.