There are a variety of forms of the cywydd, but the word on its own is generally used to refer to the cywydd deuair hirion as it is by far the most common type.
The first recorded examples of the cywydd date from the early 14th century, when it is believed to have been developed. This was the favourite metre of the Poets of the Nobility, the poets working from the fourteenth to the seventeenth centuries, and it is still used today.
The cywydd consists of a series of seven-syllable lines in rhyming couplets, with all lines written in cynghanedd. One of the lines must finish with a stressed syllable, while the other must finish with an unstressed syllable. The rhyme may vary from couplet to couplet, or may remain the same. There is no rule about how many couplets there must be in a cywydd.
The cywydd deuair hirion and the related cywydd deuair fyrion, cywydd llosgyrnog and the awdl-gywydd all occur in the list of the twenty four traditional Welsh poetic meters adopted in the later Middle Ages.