-aŭ

Etymology
. -aŭ was the only possible vocalic ending left in the language, after the dedicated part-of-speech suffixes, apart from the uncommon -eŭ. There are, however, sporadic parallels in other languages, such as Lithuanian tačiau "however".

Suffix

 * 1) A neutral suffix that does not specify the part of speech of a word. Generally used for words such as hodiaŭ "today" which may be either an adverb or a conjunction, noun, preposition, or adjective, depending on context.

Usage notes
In theory the suffix -aŭ may be replaced by another grammatical suffix, or simply dropped, but in practice this does not occur outside poetry:
 * "aŭ does not belong to the root, but is only a conventional ending, and is thus as easily left off as the o in nouns." Zamenhof further wrote of the "neutral but definite ending aŭ", that is, that -aŭ does not define the part of speech of the root. He suggested in 1892 that it could be replaced with an apostrophe. —Kalocsay & Waringhien 1985:121.

Instead, grammatical and other suffixes, additional roots, etc., are appended to the -aŭ ending, e.g. ĉirkaŭ → ĉirkaŭo, ĉirkaŭi, ĉirkaŭa, ĉirkaŭaĵo, ĉirkaŭpreni, etc.

Derived terms
-aŭ is not a productive suffix, and is only found with a few roots. These are:



Of the adverbs, only can be used in the comparative. The preposition, the numeral and the neologistic interjections  and  are bare roots and do not contain this suffix.