Jamieson, Fausset and Brown on 1 Chronicles 21:20-21

20-21. Ornan was threshing wheat--If the census was entered upon in autumn, the beginning of the civil year, the nine and a half months it occupied would end at wheat harvest. The common way of threshing corn is by spreading it out on a high level area, and driving backwards and forwards upon it two oxen harnessed to a clumsy sledge with three rollers and some sharp spikes. The driver sits on his knees on the box, while another person is employed in drawing back the straw and separating it from the grain underneath. By this operation the chaff is very much chopped, and the grain threshed out.