If the double_sided parameter is set to true and insert_blank_page is passed through the address_placement parameter, the letter content will start directly on the back of the first page (i.e. address page). The letter structure will be as follows:
Sheet 1
front = address information
back = letter content
Sheet 2
front = letter content
back = letter content
.. etc
In order for the content to begin on the front of Sheet 2, a blank page may be added as part of the letter.