I have noticed that my parish quite often departs from the lectionary for the responsorial psalm. I wasn’t sure if that was a problem (after all, the gradual may be more than just another reading, but it is another reading, and no one would dream of making a substitution for the gospel, right?), occasioning a trip to the missal.
So far as the rubrics are concerned, it turns out that substitutions are licit—within certain bounds. GIRM 61 explains that while the responsorial psalm supplied by the lectionary is not obligatory, it should be used as a rule, and when a substitution is made, the choice must (1) be the proper or seasonal antiphon and Psalm from the Lectionary, or an antiphon and Psalm from another collection thereof which has been approved by USCCB or the local ordinary, and (2) correspond to the other readings.
This stops far short of free license for liturgical improv, but its terms are still quite broad. Here’s my question: Do any of you know of any CDW instructions (or any other helpful guidance, for that matter) which guides a parish’s exercise of discretion?