Welsh hymn tunes were simple, rousing melodies which could be easily grasped by congregations who often were hearing them for the first time. Also, the tunes were often named after the places where they were written, so that peripatetic organists could instantly recognise the tune and let the chapel sing whatever lyrics they wanted. 'Cwm Rhondda' simply means 'the Rhondda Valley.' However, despite its Welsh name, the hymn is actually written, and properly performed, in English. 'Cwm Rhondda' was written by John Hughes, the organist of Capel Rhondda in Hopkinstown, near Pontypridd. Hughes had been asked to write a tune for the inauguration of the chapel and its organ in the early 1900s. He took an earlier set of words written by William Williams (Arglwydd, Arwain Trwy'r Anialwch), who published them in 1745, and arranged a tune to them. Unusually, there had been a large influx of English-speakers into that area as the railway was being built and the chapel was originally intended to serve as a place of worship for them. Hughes used an English translation, 'Guide Me, O Thou Great Jehovah,' for its first performance in 19073, with Hughes himself at Capel Rhondda's mighty organ and it rapidly gained in popularity.

from the BBC web site