Yet there seems to be no due diligence in thoroughly examining all the primary sources. This means not only the Confession and the Catechisms, but also the Directory for Public Worship (DPW) and the Form of Presbyterial Church Government (FPCG) along with the Minutes of the Assembly and what eventually became the Scottish Psalter 1650, or the Assembly's revision of Rouse's psalter over and above Barton's.
In other words, to suggest that "psalms" in the Westminster Standards means something other than the psalms, hymns and songs of David, Asaph and Korah in the Old Testament's Sepher Tehillim or Book of Praise, is either disingenuous or incompetent to the question. That psalmody may or may not be popular these days in or outside the P&R church is again, beside the question as to what the Westminster Assembly actually taught according to the primary sources. Likewise whatever the common use or meaning of the term "psalm" might be, whether today or in the Assembly's day is immaterial; the Assembly's use pre-empts the common usage, if not dictates how we are to understand the term, at least when it comes to the Westminster Standards, the animus imponentis of contemporary presbyterian churches notwithstanding when it comes to their affirmation of the WCF.
In other words, let there be no mistake about it. The overwhelming, if not unanimous use of the term in the Standards, along with the Minutes and the Assembly's Psalter, categorically refers to the 150 Old Testament psalms. The same, written in part by David, "the sweet psalmist of Israel" who said "The Spirit of the LORD spake by me, and his word was in my tongue. (2 Sam. 23:1,2)".
And, because singing of psalms is of all other the most proper ordinance for expressing joy and thanksgiving, let some pertinent psalm or psalms be sung for that purpose, before or after reading some portion of the word suitable to the present business...
The sermon ended, let him not only pray. . . And so, having sung another psalm, suitable to the mercy, let him dismiss the congregation with a blessing. . . When the congregation shall again be assembled, the like course in praying, reading, preaching, singing of psalms, and offering up more praise and thanksgiving, that is before directed for the morning, is to be renewed and continued, so far as the time will give leave . . . (emph. added)
It is the duty of Christians to praise God publickly, by the singing of psalms together in the congregation, and also privately in the family.
In singing of psalms, the voice is to be tunably and gravely ordered; but the chief care must be to sing with understanding, and with grace in the heart, making melody unto the Lord.
That the whole congregation may join herein, every one that can read is to have a psalm book; and all others, not disabled by age or otherwise, are to be exhorted to learn to read. But for the present, where many in the congregation cannot read, it is convenient that the minister, or some other fit person appointed by him and the other ruling officers do read the psalm, line by line, before the singing thereof (emph. added).
That the whole congregation may join herein, every one that can read is to have a psalm book; and all others, not disabled by age or otherwise, are to be exhorted to learn to read.
Besides publick reading of the holy scriptures, every person that can read, is to be exhorted to read the scriptures privately, (and all others that cannot read, if not disabled by age, or otherwise, are likewise to be exhorted to learn to read,) and to have a Bible.
A few verbal alterations were suggested by the House of Lords and adopted by the Commons. The most important of these was, that to the direction in the section of singing of Psalms 'that every one that can read is to have a Psalmbook.' their Lordships proposed to add the words, 'and to have a Bible.' The Commons, improving on the suggestion, proposed to transfer the words to the section of the public reading of the Scriptures and developed them into a paragraph similiar in form to the one in the section of singing of Psalms (p.217).
[B]ecause the original tongues [of Hebrew and Greek] are not known to all the people of God, who have right unto, and interest in the Scriptures, and are commanded, in the fear of God, to read and search them, therefore they are to be translated into the vulgar language of every nation unto which they come, that the Word of God dwelling plentifully in all, they may worship Him in an acceptable manner; and, through patience and comfort of the Scriptures, may have hope (emph. added).Consequently the common practice at the Reformation was that Reformed churches not only translated the Scripture into the vernacular tongue, but also the Psalms in a metrical or rhyming psalter that men might 'worship God in an acceptable manner'.
Ordered – That whereas the Honble House of Commons hath, by an order bearing the date the 20th of November 1643, recommended the Psalms set out by Mr. Rouse to the consideration of the Assembly of Divines, the Assembly hath caused them to be carefully perused, and as they are now altered and amended, do approve of them, and humbly conceive that if may be useful and profitable to the Church that they be permitted to be publicly sung (Minutes and Papers of the Westminster Assembly 1643-1652 , 2011, Van Dixhoorn, III: 708)
That whereas on the 14th of November 1645, in obedience to an order of this Honourable House concerning the said Mr. Barton's Psalms, we have already recommended to this Honourable House one translation of the Psalms in verse, made by Mr. Rouse, and perused and amended by the same learned gentleman, and the Committee of the Assembly, as conceiving it would be very useful for the edification of the Church in regard it is so exactly framed according to the original text: and whereas there are several other translations of the Psalms already extant: We humbly conceive that if liberty should be given to people to sing in churches, every one the translation which they desire, by that means several translations might come to be used, yea, in one and the same congregation at the same time, which would be a great distraction and hindrance to edification (Minutes and Papers of the Westminster Assembly 1643-1652, 2011, Van Dixhoorn, V: 303).
After the Confession of Faith and the Shorter Catechism, the best known piece of work by the Divines is what is usually called the "Scotch Metrical Psalms." They were neither originated, nor were they finally completed by the Assembly, but it was due to their adoption by that body that they came, as a part of the proposed uniformity of worship, to be used in Scotland, and their singing by the Covenanters endeared them to the heart of that nation (pp.161-8).
One of the sections of the Directory is given to the Singing of Psalms, and declares it "the duty of Christians to praise God publickly, by Singing of Psalms together in the Congregation, and also privately in the family." This rubric manifestly implied the provision of a Psalm Book, and it was made part of the function of the Assembly in preparing a basis for uniformity in worship in the Churches of the three kingdoms, to supply them with a common Psalm Book (p.52, emph. added.).