Cyfrinach Beirdd Ynys Prydain (1829)
'Cyfrinach Beirdd Ynys Prydain' is a highly original metrical treatise and reveals that Iolo was indeed the main authority of his generation on Welsh metrics. He combined authority with ingenuity, for his particular classification was structured along guidelines established by the botanist Linnaeus.
The Welsh metres which had been classified and standardized by Dafydd ab Edmwnd (fl. 1450–97)
By 1791 the so-called Glamorgan system was at the heart of Iolo's bardo-druidic vision of the past, and conferred ancient authority (and thence superiority) on the literary tradition of his native county. He now argued that 'Cyfrinach Beirdd Ynys Prydain' was proof that the poets of north Wales had fallen away from the true bardic path and descended into barbarism, while the genuine tradition of Bardism prevailed in south Wales under the aegis of the Glamorgan bards.