The dividing line between east and west longitude is the Prime Meridian. It's an imaginary line that runs from the North Pole to the South Pole, passing through the Royal Observatory in Greenwich, England.

* East longitude is measured in degrees east of the Prime Meridian.

* West longitude is measured in degrees west of the Prime Meridian.

Think of it like a giant clock face with the Prime Meridian as the 12 o'clock line. Everything to the right (east) of the Prime Meridian is east longitude, and everything to the left (west) is west longitude.