16. While drawn up in terms less exacting than those employed for the prayers of the Liturgy, devotional prayers and formulae should be inspired, nonetheless, by Sacred Scripture, the Liturgy, the Fathers of the Church and the Magisterium, and concord with the Church’s faith. The established public prayers attached to pious devotions and the various acts associated with pious exercises must always be approved by the local Ordinary(Cf. canon law 826.3).
Ease of participation. Familiarity.
The full document, the Directory on Popular Piety and the Liturgy, is online at the Vatican site.