Part 91 of the Federal Aviation Regulations contains rules governing cruising altitudes to be used by aircraft operating under visual flight rules. These rules minimize the occurrence of aircraft approaching each other head on, where they are most difficult to see.
The rules state that if an aircraft is cruising at an altitude of 3,000 or more above the ground, then the pilot must select at an odd altitude plus 500 feet if eastbound, and an even altitude plus 500 feet if westbound.
For example, an aircraft flying eastbound would use 3500, 5500, 7500, 9500, etc., while west bound altitudes are 4500, 6500, 8500, 10500, etc.
The highest VFR cruising altitude is 17500, since flight under visual flight rules is not permitted at or above 18000.
ยง 91.159 VFR cruising altitude or flight level.