Etymology
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-teen 
word-forming element making cardinal numbers from 13 to 19, meaning "ten more than," from Old English -tene, -tiene, from Proto-Germanic *tekhuniz (cognates: Old Saxon -tein, Dutch -tien, Old High German -zehan, German -zehn, Gothic -taihun), an inflected form of the root of ten; cognate with Latin -decim (source of Italian -dici, Spanish -ce, French -ze).
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-teenth 
word-forming element making ordinal numbers from 13 to 19, from -teen + -th (1), displacing Old English -teoða, -teoðe (West Saxon), related to teogoða (Anglian) "tenth."
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