Odd-even rationing is a method of rationing in which access to some resource is restricted to half the population on any given day. In a common example, cars are allowed to drive or to purchase gasoline on alternate days, according to whether the last digit in their license plate is even or odd. Similarly, during a drought, houses can be restricted from using water outdoors according to the parity of the house number.
One issue with odd-even rationing by day is that two "odd" days in a row occur whenever a month ends on an odd-numbered day.[1]
Odd–even rationing was instituted in the U. S. as part of the response to the second gas crisis in 1978, when turbulent conditions in Iraq and Iran led to worldwide oil price increases, even though a supply shortage did not materialize in the U. S.[2]
Zero is an even number; indeed, half of the numbers in a given range end in 0, 2, 4, 6, 8 and the other half in 1, 3, 5, 7, 9, so it makes sense to include 0 with the other even digits for rationing.
Nonetheless, the parity of 0 has caused confusion. The relevant legislation sometimes stipulates that zero is even.[3] In fact, an odd-even restriction on driving in 1977 Paris did lead to confusion when the rules were unclear. On an odd-only day, the police avoided fining drivers whose plates ended in 0, because they did not know whether 0 was even.[4]