In the eighteenth and nineteenth centuries, many novels were published in three volumes. Such publications were called triple-deckers. Sometimes the stories were structured to take advantage of the breaks between volumes (as in the first edition of Frankenstein, 1818), but not always.

Novels were published this way for several reasons: first, because it was difficult to bind very large works in a single volume; second, because having a single work in several volumes allowed it to circulate more rapidly at the newly developing lending libraries.

From the Guide to Literary Terms by Jack Lynch.
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