"Penillion", a special form of
the Welsh "cynghanedd"
(fixed metre poetry) was probably employed by the bards of both North and South
Wales, but has only survived in North Wales. The oldest instrumental music to
which Penillion was sung has been documented only since the 18th
century, but probably goes back to the age of the bards.
It is perhaps John Parry who describes this art the best:
“(...) The singer is obliged to follow the harper, who may change the tune, or perform variations, ad libitum, whilst the vocalist must keep time, and end precisely with the strain. The singer does not commence with the harper, but takes the strain up at the second, third or fourth bar, as best suits the pennill he intends to sing; and this is constantly done by persons, who are totally unacquainted with music!“ 
The most popular tunes for this art in the last 200 years have been the traditional Welsh harp airs such as ,Bro Gwalia‘, ,Ffawel Philip Ystwyth‘ ,Llwyn Onn‘  etc. At the end of the 19th century there were at least 46 different tunes.