Woodyates was a Post Town in Dorset halfway betwen Salisbury and Blandford Forum, which closed before April 1845. The envelope below was from Thickthorne which was the "No.1" receiving house in the Woodyates Penny Post, and was sent as Free in December 1839, after the introduction of the Uniform Fourpenny Post. Because it was sent free there was only the 1d Penny Post to pay, the letter being free in the General Post.
This was an early use of the envelope as prior to the introduction of Uniform Fourpenny Post the postage was was based on the number of sheets of paper (and an envelope counted as a sheet, effectively doubling the cost of sending a letter).