There is an actual chair - the "Cathedra Petri" - within the reliquary. It is an ancient chair, described in the Catholic Encyclopedia in this way: "The seat is about one foot ten inches above the ground, and two feet eleven and seven-eighths inches wide; the sides are two feet one and one-half inches deep; the height of the back up to the tympanum is three feet five and one-third inches; the entire height of the chair is four feet seven and one-eighth inches. According to the examination then made by Padre Garucci and Giovanni Battista de Rossi, the oldest portion is a perfectly plain oaken arm-chair with four legs connected by cross-bars. The wood is much worm-eaten, and pieces have been cut from various spots at different times, evidently for relics. To the right and left of the seat four strong iron rings, intended for carrying-poles, are set into the legs."
Pictured here is the Bernini reliquary, and the ancient chair within it. Much of the chair is oak, and not part of the original; however, there are several pieces of acacia wood incorporated, and it is quite certain that these pieces are part of the chair upon which St. Peter sat as he taught.