Link: Framework agreement relating to the Sovereign Grant
Privy Purse and Duchy of Lancaster
This is a historical term used to describe income from the Duchy of Lancaster, which is used to meet both official and private expenditure by The Queen.
The Duchy of Lancaster is a portfolio of land, property and assets held in trust for the Sovereign in his/her role as Sovereign. It is administered separately from the Crown Estates.
Its main purpose is to provide an independent source of income, and is used mainly to pay for official expenditure not met by the Sovereign Grant (primarily to meet expenses incurred by other members of the Royal Family).
There's also a section regarding the Sovereign's personal wealth and income.
royal finances
Normally, there shouldn't be any occasion to reimburse the Sovereign's Grant (notable exception being the costs for renovating Frogmore Cottage), because the people administering the Sovereign's Grant wouldn't be disbursing funds for expenses not covered by the Grant. The monarch would pay non-Grant covered expenses directly from the Duchy of Lancaster income or from their private income and wealth.
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