Interchanges in all states are numbered to reduce motorist confusion and to enable motorists to determine distances between exits within a state. Normally, one of two exit numbering methodologies is used: mileage-based or sequential. Mileage-based exit numbering systems have exit numbers that correspond to the nearest milepost, with the zero milepost normally at the southern or western end of the route. Sequential exit numbering simply starts with exit one and counts upward in a northerly or easterly direction. The mileage-based numbering system is preferred since it both allows motorists to easily reference the distance between two exits by calculating the difference between the two exits while also not upsetting the preexisting interchange numbers when a new interchange is constructed. If more than one exit is within the same milepost zone, then the exits are identified by letter suffixes (A, B, C, etc.), with the lowest letter (A) being applied to the exit that is the furthest to the west or south. Some states used to implement geographical lettering patterns, such as E/W or N/S to identify specific exits, but that convention is mostly phased out with some exceptions in the Northeast. FHWA mandates that all states use mileage based exit numbers by 2020, so eventually sequential based systems will be of historical nature only.