Royal wills are never made public.  It means what happens to much of the Queen’s personal wealth after her death last week will remain a family secret.   

  Forbes estimated last year that the late monarch’s personal fortune was worth $500 million, consisting of jewelry, an art collection, investments and two residences, Balmoral Castle in Scotland and Sandringham House in Norfolk.  The Queen inherited both properties from her father, King George VI.   

  “[Royal wills] they’re hidden, so we have no idea really what they have and what they’re worth, and that’s never been made public,” Laura Clancy, a lecturer in media at Lancaster University and author of a book on royal finances, told CNN Business.   

  But the vast majority of the royal family’s wealth — totaling at least 18 billion pounds ($21 billion) in land, property and investments — is now passing on a well-trodden, centuries-old path to the new monarch, King Charles and his heir.  .   

  The line of succession makes Prince William, now first in line to the British throne, a much wealthier man.   

  The future king inherits the private estate of the Duchy of Cornwall from his father.  The duchy has an extensive land and property portfolio covering almost 140,000 acres, most of it in the south west of England.   

  Created in 1337 by King Edward III, the estate is worth about 1 billion pounds ($1.2 billion), according to its accounts for the last financial year.   

  Income from the estate “is used to fund the public, private and charitable activities” of the Duke of Cornwall, its website says.  This title is now held by Prince William.   

  By far the largest chunk of the family’s fortune, the £16.5 billion ($19 billion) Crown Estate, is now owned by King Charles as reigning monarch.  But under an agreement dating back to 1760, the monarch hands over all profits from the estate to the government in exchange for a piece, called the Sovereign Grant.   

  The estate includes vast tracts of property in central London and the hinterland around England, Wales and Northern Ireland.  It has company status and is run by a chief executive and commissioners — or non-executive directors — appointed by the monarch on the recommendation of the prime minister.   

  In the last financial year, it generated a net profit of almost 313 million pounds ($361 million).  Of this, the UK Treasury paid the Queen a grant of £86 million ($100 million).  This equates to £1.29 ($1.50) per person in the UK.   

  Most of this money is spent on maintaining the royal family’s properties and paying their staff.   

  The Sovereign Grant is usually equivalent to 15% of the estate’s profits.  But in 2017, the payment was increased by up to 25% over the next decade to help pay for renovations at Buckingham Palace.   

  King Charles also inherits the Duchy of Lancaster, a private estate dating back to 1265, which was valued at around 653 million pounds ($764 million) according to his most recent accounts.  Income from her investments covers official expenses not covered by the Sovereign Grant and helps support other members of the royal family.   

  Despite the huge sums, the monarch and his heir are limited in how much they can personally benefit from their wealth.   

  The King can only spend the Sovereign’s Grant on royal duties.  And neither he nor his heir is allowed to profit from the sale of assets in their duchies.  Any profit from disposals is reinvested back into the property, according to an explanation published by the Institute of Governments (IfG).   

  The UK Treasury must also approve all major real estate transactions, the IfG said.   

  However, unlike the Sovereign Grant created by the Crown Estate, both duchies are private sources of wealth, meaning their owners are not required to provide details beyond reporting their income, IFG said.   

  Last year, King Charles, then Duke of Cornwall, paid himself £21 million ($25 million) from the Duchy of Cornwall estate.   

  Neither Prince William nor King Charles are required to pay any form of tax on their estates, although the two dukedoms have voluntarily paid income tax since 1993, according to the IfG.   

  The move came a year after the royal family faced heavy criticism for planning to use public money to repair Windsor Castle, which had been damaged in a fire, Clancy said.   

  “Of course, voluntary income tax [is] it is not a fixed rate and they do not have to declare how much income they make on their taxes.  So it’s actually like pulling a figure out of thin air,” Clancy said.   

  Buckingham Palace did not immediately respond to CNN Business when asked for comment.