This paper is a cross-linguistic study of dual number forms in the world's languages. According to Plank (1989; "On Humboldt on the dual"), it marks the beginning of a new era in comparative grammar research, because it is the first work that focuses on a particular grammatical phenomenon in many different languages, rather than on classifying entire languages. (The 1828 version is a kind of preprint; it was officially published in 1830, in the "Abhandlungen der historisch-philologischen Klasse der Königlichen Akademie der Wissenschaften aus dem Jahre 1827, 161-187".)