Why do some adherents to the Westminster Standards sing hymns, rather than just psalms?

The [Westminster Confession of Faith, 21.5](http://www.opc.org/wcf.html#Chapter_21) , in its list of elements that are "parts of the ordinary religious worship of God," includes the: > singing of psalms with grace in the heart The Confession makes no mention of the singing of *hymns* or other songs, however. Thus some adherents to the Westminster Standards, like the [RPCNA](https://en.wikipedia.org/wiki/Reformed_Presbyterian_Church_of_North_America) , argue that this indicates a position of "exclusive psalmody," that is, that only selections from the canonical book of Psalms should be sung in worship services. However, this section of the Westminster Confession is also approved by other Presbyterian denominations, like the [PCA](https://en.wikipedia.org/wiki/Presbyterian_Church_in_America) and the [OPC](https://en.wikipedia.org/wiki/Orthodox_Presbyterian_Church) , which reject exclusive psalmody. Their worship services often include the singing of psalms, but not exclusively – they also sing hymns and, in some congregations, contemporary praise songs. The PCA and OPC both use a version of the Westminster Standards that differs in some respects from the one approved by the Westminster Divines in the 17th century. So why did they not also change this line to more clearly indicate their belief that the singing of other songs is acceptable? That is: **What is the basis for understanding the Westminster Standards as permitting hymns and other non-psalms in worship?**