213. Adverbs, Prepositions, Conjunctions, and Interjections are called Particles. In their origin adverbs, prepositions, and conjunctions are either (1) case forms , actual or extinct, or (2) compounds and phrases. Particles cannot always be distinctly classified, for many adverbs are used also as prepositions and many as conjunctions (§ 219 and § 222).
Meagan Ayer, Allen and Greenough’s New Latin Grammar for Schools and Colleges. Carlisle, Pennsylvania: Dickinson College Commentaries, 2014. ISBN: 978-1-947822-04-7. http://dcc.dickinson.edu/grammar/latin/particles