in the 17th century, the region's vineyards expanded, and the earliest known mention of "port wine" dates from 1675. the methuen treaty between portugal and england in 1703, and the subsequent establishment of many british port lodges in porto meant that port wine became the primary product of the region, and it became economically very important to portugal. as part of the regulation of the production and trade of this valuable commodity, a royal portuguese charter of 10 september 1756 defined the production region for port wine. it thus became the world's first wine region to have a formal demarcation. the vineyards covered by this demarcation were situated in the western part of the present region. later, the vineyards have progressively expanded to the east into hotter and drier areas.