The ceiniog (Latin: denarius; English: penny; plural: ceiniogau) was the basic currency of the medieval Welsh kingdoms such as Gwynedd and Deheubarth. Hywel Dda was the only ruler recorded as minting his own proper coins; however, the ceiniog was not a coin but a value of silver. The "legal penny" (Latin: denarius legalis; Welsh: ceiniog cyfreith) was the weight of 32 wheat grains in silver; the "curt penny" (Welsh: ceiniog cwta), the weight of 24 wheat grains. The latter was based on the old Roman pound; the former, Charlemagne's and Offa's.^[1] The Welsh half-penny was the dymey of 12 wheat grains (roughly ⅓ the "legal penny")^[2] and the farthing (quarter-penny) was the firdlyc of 6.^[3]

Since the value in ceiniogau of most common goods and animals were regulated by the Laws of Hywel Dda, the system also simplified barter in Wales.