The lines that run north and south but measure east-west from the Prime Meridian are lines of longitude.

Here's why:

* Lines of longitude are imaginary lines that run from the North Pole to the South Pole, passing through all points of equal longitude.

* The Prime Meridian is the zero-degree line of longitude, and all other lines of longitude are measured east or west of it.

* Lines of latitude run east to west, parallel to the equator, and measure distances north or south of the equator.

So, while lines of longitude run north-south, they are measured in degrees east or west of the Prime Meridian, which is why they are used to determine locations on the Earth in an east-west direction.