Drawn up as they were before maps I always imagine they were described as "from this rock to that hill" etc, but maybe not.
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The red lines denote the hundred boundaries - taken from Andrews' and Drury's Map of Wiltshire 1773 - Underditch Hundred runs up from the lower right.
More on the hundred of Underditch: Introduction', A History of the County of Wiltshire: Volume 6 (1962), pp. 195-198. URL: http://www.british-history.ac.uk/report.aspx?compid=41810