Altar Cards

To assist the memory of the celebrant at Mass in those prayers which he should know by heart, cards on which these prayers are printed are placed on the altar in the middle, and at each end. They were not used before the sixteenth century, and even at present are not employed at the Mass celebrated by a bishop, who reads all the prayers from the Pontifical Canon. At the time that Pius V revised the Missal, only the card at the middle of the altar was used, and it was called the "Tabella Secretarum" (tit. xx). 

Later, another was added containing the Gospel of St. John (recited usually at the end of Mass), and placed on the Gospel side. For the sake of symmetry, another containing the prayer "Deus qui humanae substantiae", which is said by the celebrant when he blesses the cruet of water, and the psalm "Lavabo", recited at the washing of the hands, was placed on the Epistle side. 

Only during Mass should the cards stand on the altar, the middle one resting against the crucifix or tabernacle, the side ones against the candlesticks or superstructural steps of the altar. At any other time they are either removed or placed face downwards on the altar under the altar cover. When the Blessed Sacrament is exposed outside of Mass, the cards must be removed (Cong. Sac. Rit., 20 December, 1864). If these cards are framed, the frames should, as far as possible, correspond to the architecture of the altar.[1]