This dissertation examines how borrowed derivational morphemes such as -age, -ity, -cion, and -ment became productive in the English language, particularly in the fourteenth through sixteenth centuries. It endeavors to expand our current understanding of morphological productivity as a historical phenomenon---to account for not only aggregate quantitative measures of the products of morphological processes, but also some of the linguistic mechanisms that made those processes more productive for language users. Judgments about the productivity of different suffixes in the late ME period cannot be made on counts of frequency alone, since the vast majority of uses were not neologisms or newly coined hybrid forms but rather borrowings from Latin and French. It is not immediately clear to the historical linguist if Middle English speakers perceived a derivative such as enformacion as an undecomposable word or as a morphologically complex word. By examining usage patterns of these derivatives in guild records, the Wycliffite Bible, end-rhymed poetry, medical texts, and personal correspondence, this project argues that several mechanisms helped contribute to the increased transparency and perceived productivity of these affixes. These mechanisms include the following: the use of rhetorical sequences of derivatives with the same base or derivatives ending in the same suffix; the frequent use of derivatives as end rhymes in poetry; the lexical variety of derivatives ending in the same suffix; and the more frequent use of certain bases compared to their derivatives. All of these textual and linguistic features increased readers' and listeners' ability to analyze borrowed derivatives as suffixed words. Ultimately, the dissertation finds that several borrowed affixes were seen as potentially productive units of language in the late ME period, though some were seen as more productive than others in different discourses and contexts. It also emphasizes the value of register studies for understanding the specific motivations for the use of borrowed derivatives in different discourses, as well as the morphological consequences of salient usage patterns within different registers.