Definition - What does Grandfather Clause mean?
A grandfather clause is a clause in a piece of legislation which allows for exemptions for certain laws or rules to be made due to circumstances that existed prior to the legislation. For example, a town government may create new legislation which states that all buildings in the town must be under five stories tall. However, the legislation may include a grandfather clause which allows for buildings that have already been built, and are over five stories tall, to remain.
Justipedia explains Grandfather Clause
The purpose of a grandfather clause is to provide exemptions to new laws due to existing realities that require such exemptions. Grandfather clauses are often used when it is simply impractical to do otherwise. For example, a certain area of a river may be designated by a local government for strictly recreational purposes. However, if a bridge is already built over that section of the river, then it may be impractical to enforce the law to the fullest extent possible. That could mean knocking down the bridge. Instead, the government could include a grandfather clause which allows the bridge to be an exemption to the rule since it already existed before the new legislation.