ChapterIII-I. Psalms and Their Connection With Christian Prayer
100. In the liturgy of the hours the Church in large measure prays through the magnificent songs that the Old Testament authors composed under the inspiration of the Holy Spirit. The origin of these verses gives them great power to raise the mind to God, to inspire devotion, to evoke gratitude in times of favor, and to bring consolation and courage in times of trial.
101. The psalms, however, are only a foreshadowing of the fullness of time that came to pass in Christ the Lord and that is the source of the power of the Church's prayer. Hence, while the Christian people are all agreed on the supreme value to be placed on the psalms, they can sometimes experience difficulty in making this inspired poetry their own prayer.
102. Yet the Holy Spirit, under whose inspiration the psalms were written, is always present by his grace to those believers who use them with good will. But more is necessary: the faithful must "improve their understanding of the Bible, especially of the psalms,"  according to their individual capacity, so that they may understand how and by what method they can truly pray through the psalms.
103. The psalms are not readings or prose prayers, but poems of praise. They can on occasion be recited as readings, but from their literary genre they are properly called Tehillim ("songs of praise") in Hebrew and psalmoi ("songs to be sung to the lyre") in Greek. In fact, all the psalms have a musical quality that determines their correct style of delivery. Thus even when a psalm is recited and not sung or is said silently in private, its musical character should govern its use. A psalm does present a text to the minds of the people, but its aim is to move the heart of those singing it or listening to it and also of those accompanying it "on the lyre and harp."
104. To sing the psalms with understanding, then, is to meditate on them verse by verse, with the heart always ready to respond in the way the Holy Spirit desires.
The one who inspired the psalmist will also be present to those who in faith and love are ready to receive his grace. For this reason the singing of psalms, though it demands the reverence owed to God's majesty, should be the expression of a joyful spirit and a loving heart, in keeping with their character as sacred poetry and divine song and above all with the freedom of the children of God.
105. Often the words of a psalm help us to pray with greater ease and fervor, whether in thanksgiving and joyful praise of God or in prayer for help in the throes of suffering. But difficulties may arise, especially when the psalm is not addressed directly to God. The psalmist is a poet and often addresses the people as he recalls Israel's history; sometimes he addresses others, including subrational creatures. He even represents the words as being spoken by God himself and individual people, including, as in Ps 2, God's enemies. This shows that a psalm is a different kind of prayer from a prayer or collect composed by the Church. Moreover, it is in keeping with the poetic and musical character of the psalms that they do not necessarily address God but are sung in God's presence. Thus St. Benedict's instruction: "Let us reflect on what it means to be in the sight of God and his angels, and let us so stand in his presence that our minds are in harmony with our voices." 
106. In praying the psalms we should open our hearts to the different attitudes they express, varying with the literary genre to which each belongs (psalms of grief, trust, gratitude, etc.) and to which biblical scholars rightly attach great importance.
107. Staying close to the meaning of the words, the person who prays the psalms looks for the significance of the text for the human life of the believer.
It is clear that each psalm was written in its own individual circumstances, which the titles given for each psalm in the Hebrew psalter are meant to indicate. But whatever its historical origin, each psalm has its own meaning, which we cannot overlook even in our own day. Though the psalms originated very many centuries ago among an Eastern people, they express accurately the pain and hope, the unhappiness and trust of people of every age and country, and sing above all of faith in God, of revelation, and of redemption.
108. Those who pray the psalms in the liturgy of the hours do so not so much in their own name as in the name of the entire Body of Christ. This consideration does away with the problem of a possible discrepancy between personal feelings and the sentiments a psalm is expressing: for example, when a person feels sad and the psalm is one of joy or when a person feels happy and the psalm is one of mourning. Such a problem is readily solved in private prayer, which allows for the choice of a psalm suited to personal feelings. The divine office, however, is not private; the cycle of psalms is public, in the name of the Church, even for those who may be reciting an hour alone. Those who pray the psalms in the name of the Church nevertheless can always find a reason for joy or sadness, for the saying of the Apostle applies in this case also: "Rejoice with the joyful and weep with those who weep" (Rom 12:15). In this way human frailty, wounded by self-love, is healed in proportion to the love that makes the heart match the voice that prays the psalms. 
109. Those who pray the psalms in the name of the Church should be aware of their full sense (sensus plenus), especially their Messianic sense, which was the reason for the Church's introduction of the psalter into its prayer. This Messianic sense was fully revealed in the New Testament and indeed was affirmed publicly by
Christ the Lord in person when he said to the apostles: "All that is written about me in the law of Moses and the prophets and the psalms must be fulfilled" (Lk 24:44). The best-known example of this Messianic sense is the dialogue in Matthew's Gospel on the Messiah as Son of David and David's Lord,  where Ps 110 is interpreted as Messianic.
Following this line of thought, the Fathers of the Church saw the whole psalter as a prophecy of Christ and the Church and explained it in this sense; for the same reason the psalms have been chosen for use in the liturgy. Though somewhat contrived interpretations were at times proposed, in general the Fathers and the liturgy itself had the right to hear in the singing of the psalms the voice of Christ crying out to the Father or of the Father conversing with the Son; indeed, they also recognized in the psalms the voice of the Church, the apostles, and the martyrs. This method of interpretation also flourished in the Middle Ages; in many manuscripts of the period the Christological meaning of each psalm was set before those praying by means of the caption prefixed. A Christological meaning is by no means confined to the recognized Messianic psalms but is given also to many others. Some of these interpretations are doubtless Christological only in an accommodated sense, but they have the support of the Church's tradition.
On the great feasts especially, the choice of psalms is often based on their Christological meaning and antiphons taken from these psalms are frequently used to throw light on this meaning.