The Liber Usualis is a book of commonly used Gregorian chants in the Catholic tradition, compiled by the monks of the Abbey of Solesmes in France. According to Willi Apel, the Liber Usualis originated in the 11th century. Another source states that it was first published in the 16th century.
This 1,900-page book contains most versions of the ordinary chants for the Mass (Kyrie, Gloria, Credo, Sanctus, and Agnus Dei), as well as the common chants for the Divine Office (daily prayers of the Church) and for every commonly celebrated feast of the Church Year (including more than two hundred pages for Holy Week alone). The "usual book" or "common book" also contains chants for specific rituals, such as baptisms, weddings, funerals, ordinations, and benediction. This modal, monophonic Latin music has been sung in the Catholic Church since at least the sixth century  and through to the 2000s. An extensive introduction explains how to read and interpret the medieval musical notation (square notation of neums or neumes). A complete index makes it easy to find specific pieces.
The Liber was first edited in 1896 by Solesmes Abbot Dom André Mocquereau (1849–1930). Its use has decreased since the Second Vatican Ecumenical Council (opened by Pope John XXIII in 1962), in the constitution on the liturgy (Sacrosanctum Concilium), allowed the local language to be used in Church rites, even though the same council mandated that Gregorian Chant should retain "pride of place" in the liturgy (Sacrosanctum Concilium, 116). Gregorian chants are still sung in most monasteries and some churches, and in performances by groups dedicated to its preservation. In recent years, due to a resurgence of interest in Gregorian chant and the Tridentine Mass, some editions of the Liber Usualis have been reprinted or scanned and made available for download.
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- Solesmes Abbey
- Liber Usualis (1924, Modern notation with a five line staff, publisher number 780c) in PDF format
- Liber Usualis (1961, Solesmes notation with a four line staff) in PDF format (115 MB)
- Bergeron, Katherine. Decadent enchantments: the revival of Gregorian chant at Solesmes. Berkeley: University of California Press, c1998. ISBN 0-520-21008-5.
- Nova Organi Harmonia (a nearly complete organ accompaniment to the Liber Usualis)
- Apel, Willi. Gregorian Chant. Indiana University Press, 1958. p. 417