Halfway between the prime meridian and the 180-degree longitude line is a 90-degree longitude line. There are two 90-degree meridians (lines of longitude), one in each hemisphere.
All of the lines of longitude to the west of the prime meridian until 180 degrees are in the western hemisphere. Ninety degrees west is a line that passes through North America, including Lake Superior and the Yucatan Peninsula. Likewise, all the lines of longitude east of the prime meridian are in the eastern hemisphere. Ninety degrees east is a line that passes through Asia, including parts of Russia, China, Tibet and India. Another name for the line of longitude at 180 degrees is the International Date Line.