With the exception of a few inscriptions in the singular dative, two in the plural genitive and one in the singular nominative, most known inscriptions of nouns in orthodox ogham are found in the singular genitive, so it is difficult to fully describe their nominal morphology. The German philologist Sabine Ziegler, however, drawing parallels with reconstructions of the Proto-Celtic language morphology (whose nouns are classified according to the vowels that characterize their endings), limited the archaic Irish endings of the singular genitive to , , and .