Principles of the Liturgy, Volume I of this four-volume works, falls into three main sections: first, an overview of the complex history of this liturgy, tracing the development of the liturgical rites and families; a second section focuses on the structures and laws of liturgical celebration, dealing with key concepts like ‘assembly’, ‘dialogue between God and his people’ and ‘liturgical sings’; the third section explores the theology of liturgical celebration as an expression of the mystery of salvation and of the faith of the Church.