The classic Monopoly board game charges 10% interest when a mortgaged property is unmortgaged.
A player can mortgage an unimproved property to raise cash from the bank. The property’s mortgage value is paid immediately, but the property cannot collect rent while mortgaged. To restore its rent-earning status, the owner pays the mortgage value plus 10% interest.
The same 10% rate applies when a player unmortgages a property after another player has acquired it through a trade. Houses and hotels must be sold back to the bank before a property can be mortgaged, and building restrictions still matter when properties in a color group are developed.
Mortgage interest is often confused with rent. Mortgaging is a financing rule, not a fee charged to a visitor who lands on the space. The official rules also allow owners to trade mortgaged properties, subject to the mortgage payment rules.