Liber misticus (type of book containing chants, readings and prayers for the cathedral ordo, i.e. vespers, matutinum and mass, and on penitential days only, also tertia, sexta and nona) with material for Post-Pentecost time offices from 13 June (Saint Quricus) to 12 November Saint Emilianus. This manuscript has the most complete rendition of these offices. The first quire has the office of Saint Jerome (normally observed on 30 September following the office of Saint Quiricus 13 June; it is possible that this manuscript reflects a practice in which Saint Jerome was observed on 15 June, like in East Christianity. The similarity of this quire's palaeographical and codicological features with respect to that of the remaining quires are sufficient to discard the hypothesis proposed by Díaz y Díaz (1980) that this first quire originally belonged to another book. Its last quire, with only one folio, is displaced: it contains part of the offices of Saint Christina and Saint Bartholomew, which were observed at the end of July. It is very likely that it's original place was between quires 6 (ending with the office for Saint Cucuphas, on 25 July) and 7 (quire that has the office for Saint Felix of Gerona, 1 August).

It has very interesting monograms preceding each office that have a style that is not present in any of the manuscripts currently preserved at Santo Domingo de Silos: some of them have human shapes, representing the celebrated saints, other have animal or vegetable shapes.
Though the musical notation is of the vertical variety several neumes are tilted towards the right as is typical of the horizontal notation. This is also one of the manuscripts with what Brou (1955) called "intermittent notation" (i.e. when a syllable bears one note only sometimes such note is not copied).

Date: most authors place its production in the tenth century; others date it as tenth-eleventh century (Férotin, 1912; Rojo and Prado, 1929), end-tenth century to beginning-eleventh century (Boynton); eleventh century (Walker, 1998; Millares Carlo, 1999). Díaz y Díaz (1983) dates: ff. 1r-7v, as end-tenth century; the fragment in ff. 86 bis r-v as tenth century; ff. 91r-94v as eleventh century; but does not give a date for the rest of the manuscript.

Origin: the British Library (when it was British Museum) acquired it from Quaritch, which had bought it from the Abbey of Santo Domingo de Silos in the late nineteenth century. As noted before, this manuscript does not resemble any tenth or eleventh century manuscript preserved there. Boynton and Whitehill proposed San Millán as its place of origin. As Boynton ('A lost Mozarabic manuscript', p. 191) explains,'It was apparently produced in San Millán' (my italics) mainly based on the fact that 'the mass for San Millán refers to him several times as "noster" and "patronus"'. Although this attribution is possible, the musical notation, script, codicology and miniatures in this manuscript, suggest that this manuscript was produced elsewhere, and this becomes even more apparent if the whole corpus of Hispanic rite manuscripts are examined in person. San Millán was a powerful motherhouse and had a large network of convents and monasteries linked with it, all of which would call San Millán their patron. Furthermore, especially after the battle of Simancas in 939, any monastery or church in La Rioja, and the Castilla and León areas, was likely to call San Millán their patron, and there are many big scriptoria documented from this area (Tábara, Nájera, Albelda, Cardeña, Oña, etc...). Díaz y Díaz (1983) suggests that what he calls the main part of the codex is from Santo Domingo de Silos, but not from its monastery but from a place of exchange; he later (1995) adds that it has symptoms of the South of the Península.

Liturgical tradition: A.

Randel responsorial tone tradition: [La] Rioja, though "it shows a greater number of León symptoms" if compared with GB-Lbl Add MS 30851 (Randel, 1973, p. 70).