Applying the regulative principle has divided Reformed Christians among themselves for as long as they have held it, because what counts as commanded or warranted is not always obvious to read off a text. The clearest fight has been over song. Some Reformed and Presbyterian bodies hold that the only warranted matter for congregational singing is the Psalms themselves, since the Psalter is the hymnbook Scripture actually supplies, and this exclusive psalmody position has been maintained continuously by some Presbyterian denominations into the present, while most Reformed churches now sing hymns and other scriptural songs as matters the principle does not reach.
Musical instruments raised a parallel argument. Unaccompanied congregational singing was the near-universal Reformed practice for centuries, on the ground that instruments belonged to the ceremonial worship of the Old Testament temple and were never commanded for the New Testament church, and some conservative Presbyterian bodies still hold that position, while the great majority of Reformed congregations now use organs or other instruments freely.
A separate strand of the tradition, sometimes called the principle of circumstances, distinguishes elements of worship, which need warrant, from circumstances of worship, such as the time of a service or the language it is conducted in, which prudence alone may govern. Where exactly that line falls is itself argued, and the argument is the clearest evidence that the regulative principle constrains a Reformed congregation's worship without dictating a single settled form of it.