The hemispheric rule provides for vertical separation of aircraft.
All aircraft operating VFR at least 3,000 AGL and less than 18,000MSL must maintain appropriate altitude based on their direction of flight, specifically on their magnetic course. VFR Aircraft on a magnetic course of 000 -179, or easterly direction, must fly at odd thousands plus 500 feet, such as 3,500, 5,500, 7,500…17,500. VFR aircraft flying in a westerly direction or a magnetic course of 180-359 should fly at even thousands plus 500 feet, such as 4,500, 6,500, 8,500…16,500. As these aircraft are flying VFR, there is a margin of safety added that if two aircraft are at the same altitude, they should not be heading at each other.
Remember, this is based on magnetic course not heading. As such, be very cautious when heading directly north or south as others may have made a mistake.
The plus 500 feet provides for vertical separation from IFR traffic that fly on even and odd thousands.
There are some exclusions to the hemispheric rule. These exclusions include when being directed or authorized by ATC, in a holding pattern of 2 minutes or less, or turning.
14 CFR 91.159 – VFR cruising altitude or flight level.