Eli5: Why do a lot of international agreements last for exactly 99 years?

Question

Basically the title. For example, Hong Kong was under British sovereignty for 99 years, Bolivia obtained in 2010 a lease for a Peruvian port for 99 years since then, and in 1940 the UK granted to the US the rights to build military bases on some of its territories and control them for 99 years. Why exactly 99 and not 10, 50, 100 or any other “round” number? Is this a coincidence or is there some diplomatic reason for this?

Edit: UK and England are not the same lol

In: 607

Answer 1

99 year leases and land covenants used to symbolize “almost but not quite forever”- because no way would you live to 99 in the Middle Ages. The point was it would definitely outlive the person who received it, but it also definitely didn’t change hands and would revert back to the crown instead of being passed on as inheritance.