In reciting the Divine Office two points of order are to be noted: (1) the order or arrangement of offices, (2) the order or arrangement of Hours. The order of offices indicates which office is to be said on each day as laid down in the calendar. The order of the Hours points out which of the seven hours should be recited, firstly, secondly, etc., Matins, Lauds, Prime, Terce, etc. It is of obligation to observe both orders. But is it a sin to change wilfully the order of the office? It is not, if there be a reasonable cause for the change. For instance, if a priest cannot say the office proper to his diocese on a certain day, but says some other approved office, the change is not a sin. But if a priest, _ex industria_, substitute one office for another, it is _per se_ a venial sin; but if an office be said which is very much shorter than the calendar office, or if this changing or substituting be so frequent as to disturb gravely the good order of the year's offices, the sin may be (and, according to some authors, is) a mortal sin. It is asked whether a person fulfils his debt to the Church if he has recited by mistake an office other than the one assigned in the calendar of the day. Theologians teach that such a recitation fulfils the debt. The Church does not wish to impose a second recitation, and her axiom "_officium pro officio valet_" holds, provided always that the order of the psalms as laid down in the new psaltery is followed. This order is necessary always for validity. However, if the substituted office be very much shorter than the omitted office, it is advised to equalise them by reciting the psalms of Matins, This is a counsel and was not laid down by theologians as an obligation.