In linguistics, declension is the inflection of nouns, pronouns, adjectives, and articles to indicate number (at least singular and plural), case (nominative or subjective, genitive or possessive, etc.), and gender. A declension is also a group of nouns that follow a particular pattern of inflection.
Declension occurs in many of the world's languages, and features very prominently in many European languages, including Old English, but is much less prominent in Modern English. In contrast to Old English, at least 80 percent of the vocabulary of Modern English has been borrowed from foreign, mostly non-Germanic languages (especially Latin), whose systems of declension conflicted with those of Old English. The resulting compromises effectively eliminated most forms of inflection in late Middle and Modern English.
Most Modern English nouns are declined for number, to distinguish singular and plural - goose/geese, book/books, ox/oxen, child/children, medium/media, syllabus/syllabi, alumna/alumnae; but some do not change - deer/deer, sheep/sheep; and a few have dual plurals - fish/fish/fishes, and in British English penny/pennies/pence. Two 'fishes' indicates two types of fish (e.g., salmon and cod) while two 'fish' is just a raw number (i.e., may be two of the same fish or two different fish). Likewise, two 'pennies' indicates two coins, whereas two 'pence' indicates a two-penny value (i.e., one coin valued at two pence, five pence, etc., or two pennies, five pennies, etc.) Ultimately, 'pence' is a phonetic contraction - 'pennies' compressed from two to one syllable (viz., pennies > penns > pence), with "two pence" and "three pence" further compressed to "tuppence" and "thruppence" (or "tuppenny coin" and "thruppenny coin"). Words borrowed from Latin typically form their plurals in English as they do in Latin - thus, datum > data (not 'datums'), syllabus > syllabi, alumna > alumnae. By default, they also display the same gender in English as they do in Latin (datum, syllabus and alumna being neuter, masculine and feminine, respectively).
All Modern English nouns are still inflected for the genitive case, which is usually limited to expressing possession (occasionally, attribution) - Mary/Mary's, lamb/lamb's. Three days of the week still display the genitive case in ancient form without the apostrophe, which indicates omission of the letter 'e' - Tuesday, not Tu's Day; Wednesday, not Weden's Day (the second 'e' having been lost by orthographic contraction); and Thursday, not Thur's Day (though the 'e' is missing, so it's arguably half-way to the apostrophe).
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