Traditional Welsh poetic metres

The traditional Welsh poetic meters consist of 24 types of poetic meter, called Y Pedwar Mesur ar Hugain in Welsh. They are all written in cynghanedd of varying degrees of complexity. 
Although called "traditional," they were compiled – and later redefined at least once – in the Late Middle Ages and omit some of the older forms such as the "englyn milwr". Only a few of them were widely used by the professional poets ("Beirdd yr Uchelwyr"), and the use of some of the more complicated ones is confined to occasional poems of technical virtuosity dating to the end of the Middle Ages.
The twenty four traditional Welsh poetic meters are: