Some of them are now analyzed as ordinary adverbs, including the following: Some words were formed from the adverbial genitive along with an additional parasitic -t: The adverbial genitive also survives in a number of stock phrases; for example, in "I work days and sleep nights", the words days and nights are analyzed as plural nouns but are in fact derived historically from the genitive or instrumental cases of day and night.
(That they function as adverbs rather than as direct objects is clear from the rephrasing "I work during the day and sleep at night.")
This periphrastic form has variously been marked as used "particularly in isolated and mountainous regions of the southern United States"[14] and as having "a distinctly literary feel".
The adverbial suffix -erweise added to adjectives is derived from the feminine singular genitive adjective ending -er agreeing with the noun Weise 'manner'.
For example, the adverb glücklicherweise 'fortunately' can be analyzed as glücklicher Weise 'fortunate way [genitive]', i.e. 'in a fortunate way' or more explicitly ‘in a manner of good fortune’ (which also hints at the possessive role of the case).