Comes after the eighteenth in order; the ordinal number for 19.
From Old English 'nigontēoða,' formed from 'nigon' (nine) plus '-tēoða' (the ordinal suffix). The suffix '-th' has marked ordinal numbers since Anglo-Saxon times.
The suffix '-th' for ordinals is puzzling—why do we say 'fourth' but 'four'? It's a remnant of Old English grammar that survived even though our number system changed. Nineteenth keeps alive this ancient pattern from over 1,400 years ago.
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