Grandfathering is an allocation methodology to share emission permits among the different actors of a market mechanism (e.g. emission trading).
In this methodology, a well-defined amount of emission rights are allocated free of charge to operators on the basis of their respective historic activities/emissions.
See also : The Impacts of the Use of Different Benchmarking Methodologies on the Initial Allocation of Emission Trading Scheme Permits to Airlines
=> Glossary