 Those that end in "-i" and many that end in "-a" stem from the ancient first declension and have the accent on the ultimate syllable in genitive plural.The rest of those that end in "-a" originate from the ancient third declension and have formed their nominative singular from the ancient accusative singular; those nouns keep the accent unchanged in genitive plural.The nouns that end in -ος ("-os") are identical to the respective masculine nouns.