Refer to figure.
Meridians of longitude are lines that run from south to north between the poles. There is an infinite number, one of which is the “Greenwich” or “Prime meridian”. A meridian and its opposite on the other side of the Earth, its anti-meridian, combine to make a circumference around the Earth.

Longitude is the angle measured at the polar axis, between the plane of the Greenwich meridian and the local meridian.

Therefore, the minimum longitude is 000° at the Greenwich meridian and the maximum possible longitude is 180° at the Greenwich anti-meridian. It is named either “East” or “West”, depending where the meridian lies in relation to the Greenwich meridian.

Parallels of latitude are lines that run east/west. There is an infinite number of these, one of which is the Equator, half way between the poles and perpendicular to the line joining the poles.

Parallels of latitude, apart from the Equator, are not circumferences of the Earth, they represent smaller circles.

They are described by their angle, measured at the centre of the Earth, above or below the Equator. Thus, latitude of the North pole would be 90° N, and the South pole 90° S.

Therefore, the minimum latitude is 00° at the Equator and the maximum possible latitude is 90°, either North, or South.