Rationing is a deliberate attempt, frequently undertaken by governments, to allocate scarce supplies in the face of high demand. Severe rationing during the 1940s, 1950s and 1970s prompted American and European economists to study this subject and its consequences for product substitution.
Rationing does not exist in a free market because the excess demand would be countered by a rise in prices.
J Tobin, 'A Survey of the Theory of Rationing', Econometrica, vol. xx (1952) 512-53