Allen and Greenough's New Latin Grammar, section 213

Heavy Construction

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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).