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Bach used a German spoken at his time (1685-1750) which is considered as Neuhochdeutsch but it is not spoken any more today. Nevertheless it is still understood. In addition to this a lot of changes to grammar and spelling were done for rhyming so that stanzas fit to the melody. Example: O Haupt voll Blut und Wunden, voll Schmerz und voller Hohn, ...
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