Firth of Tay (63°22′S55°45′W) is a sound or firth, 12 miles (19 km) long and 6 miles (10 km) wide, extending in a northwest–southeast direction between the northeast side of Dundee Island and the east portion of Joinville Island. It merges to the northwest with Active Sound with which it completes the separation of Dundee and Joinville Islands.