Generally, thematic nouns and verbs (those with a "thematic vowel" between root and ending, usually /e/ or /o/) had a "fixed accent", which (depending on the particular noun or verb) could be either on the root or the ending. These words also had no ablaut variations within their paradigms. (However, accent and ablaut were still associated; for example, thematic verbs with root accent tended to have e-grade ablaut in the root, while those ending accent tended to have zero-grade ablaut in the root.) On the other hand, athematic nouns and verbs usually had "mobile accent", with varied between "strong forms", with root accent and full grade in the root (e.g. the singular active of verbs, and the nominative and accusative of nouns), and "weak forms", with ending accent and zero grade in the root (e.g. the plural active and all forms of the middle of verbs, and the oblique cases of nouns). Some nouns and verbs, on the other hand, had a different pattern, with ablaut variation between lengthened and full grade and mostly fixed accent on the root; these are termed "Narten stems". Additional patterns exist for both nouns and verbs. For example, some nouns (so-called "acrostatic nouns", one of the oldest classes of noun) have fixed accent on the root, with ablaut variation between o-grade and e-grade, while "hysterodynamic nouns" have zero-grade root with a mobile accent that varies between suffix and ending, with corresponding ablaut variations in the suffix.