The Knights Templar, part monastic order, part military force, lived by a firm code, or rule, which exists in differing versions. This Spanish version is a follow-up to J.M. Upton-Ward's highly successful edition of the French Rule. The introduction to this Catalan Rule, Barcelona Archivo de la Corona de Aragón, Cartes Reales, MS 3344, discusses the content, language and dating of the manuscript. It also provides background information derived from the French Rule (which the reader may require for a fuller appreciation of the text - see author note below) on the circumstances of the Knights Templar. There is a brief description of the provincial organisation of the Order with particular reference to the houses in Aragon, where it is most likely that the manuscript was used; a summary of clauses; and a concordance with de Curzon's 1886 edition of the French Rule. Compared to de Curzon's edition, the Barcelona text is incomplete, but it contains important clauses not found in other manuscripts. A partial transcription claiming to represent all the clauses without equivalents in de Curzon's edition was published in 1889, but it omitted several clauses now published here for the first time.