For the traditional Latin Mass, the law of the Church permits the singing of motets after the proper offertory, after the Elevation, and during Communion. The music of these motets must be suitable for divine worship, and their text must be in Latin and "approved by the Church" (Motu proprio of St. Pius X). Some interpret this to mean that the words of a supplementary motet at Mass must actually be taken from the liturgy, i.e. an antiphon, responsory, sequence, hymn, psalm, or canticle. Whether it is a matter of what is expressly permitted by ecclesiastical legislation or what is tolerated by local (or perhaps universal?) custom, some non-liturgical Latin hymns are in general usage even during Solemn Mass, e.g. Adeste fideles, Adoro te devote, O filii et filiae, O esca viatorum, O Sanctissima, Regina caeli jubila, and Veni, veni, Emmanuel. Some of these can be found in the supplement of the Liber usualis. Interestingly, these compositions are indexed in that book not as "hymns," but rather as "chants for Benedictions" in the English editions or "varia" in the Latin.

I doubt even the most rigorously rule-obsessed traditionalist would bat an eyelid at hearing one of the above-mentioned hymns during the offertory or Communion of a High Mass, but what are the criteria for the inclusion of such devotional Latin hymns within the liturgy? Does publication in a hymnal or prayerbook with an imprimatur constitute "words approved by the Church"? What about inclusion in the Benediction section of the Liber? Bear in mind that in the old rite, all the music at Benediction is considered non-liturgical except for the "Tantum ergo" and the "Panem de caelo" versicle and response. Has "Personent hodie" from the Lutheran Piae cantiones appeared in an officially approved collection? May one sing a motet with words from a Latin version of the psalter not used in Catholic liturgy, such as Purcell's "Jehova, quam multi sunt hostes mei"? Has anyone dared to do one of these hymns during High Mass? What about Whitacre's "Lux aurumque"? My hunch is that these last several examples should be avoided during the liturgy on account of their non-Catholic provenance, but it's difficult to articulate exactly what makes a non-liturgical Latin hymn acceptable, apart from its Catholic origin. Is this solely a matter of custom, or does inclusion in a publication with an imprimatur indicate official approval for use during liturgical services? Does anyone know of a comprehensive list of such Latin hymns and chants?