Harvesting is the gathering of any crop. In most places it's commonly done in the fall. Threshing applies only to grain crops and is the process of separating the seeds from the husks, stalks, etc., commonly known as the chaff. Today, threshing is simply one of the things done by a combine in the field. In days of yore, threshing was done after the harvest as a separate step, usually in winter (when not much else could be done). The threshing barn was a long, narrow building with a large door on each of the long sides. If the building was properly oriented, it was possible to beat the grain with a flail and the wind passing through the open doors of the building would carry the chaff outside, leaving the grain, or thresh, behind. In order to assure that none of the thresh escaped, there were small ledges built at the bottom of the two doors called, you guessed it, thresholds.
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