The Carthusians have retained some liturgical customs from the Middle Ages, a time when those customs were not specific to the Order, but were popular and kept by many monastics, members of other Religious Orders, and the Secular clergy.

One such custom is what is called the "Officium Missae" (The "Office of the Mass"). Archdale A. King and other writers identify this by another name, the "Missa Sicca" (the "Dry Mass").

For many medieval priests, the Missa Sicca was generally prayed right after the celebration of a private Mass, and the priest continued to wear the Mass vestments of the day. For the Carthusian, it was prayed by the choir monk alone, in the cell, without vestments.

It is, in essence, a recitation of the "proper" (and a few of the "ordinary") texts of one of two common Masses of the Blessed Virgin (Salve, Sancta Parens for ordinary time, and Rorate Caeli, for Advent). Among Carthusians, it has always been prayed in the cell, i.e., alone, and it was generally attached to the end of hour of Prime or Terce of the daily Office of the Blessed Virgin. Today, its use is optional. In fact, the entire Office of the Blessed Virgin may be replaced, with the Superior's permission (the Prior or Prioress of a monastery) with another form of devotion to the Blessed Virgin. (However, it seems that many, if not most, Carthusians keep the original custom, and today, this includes lay Brothers as well as Choir religious). The Officium De Beata can be found at Joseph Hoffman's page and can be downloaded.

The "Agenda" is the Carthusian Office of the Dead. Prior to the Second Vatican Council, the office was said in choir and followed by the Canonical Office (the office of the day). Today, it is prayed once a week in the cell (alone) by the Fathers. Very little has changed, except that additional choices for readings have been included. The Office is said in the church Choir on November 2nd (All Souls), on November 13th (for departed members of the Order and families and relatives), and when a monk or nun of a house dies.