trefn yr wyddor

== Welsh == === Noun === trefn yr wyddor alphabetical order === Further reading === R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke, et al., editors (1950–present), “trefn yr wyddor”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies