First, a little background on estates in land. The simplest is what is called a "fee simple". When you have a fee simple interest in some land it is what we traditionally think of as owning the land. That land is yours to do anything you want with (that's legal) from when you acquired the interest until the end of time or until you convey your interest to someone else.

Another common estate in land is the leasehold estate. If I own some land in fee simple I can sell you a leasehold estate that runs for a specific time period. During that time period it is essentially your land. When the leasehold ends my fee simple ownership resumes.

Another common estate is the life estate. This is a lot like a leasehold except that rather than being for some fixed time period it is for the life of someone. They were common in wills (I have no idea if they still are common), typically with someone leaving their estate to the widow for life with the remainder (the term for what comes after any finite estates like life estates end) to the eldest son or split among their children or some such.

Note that in the above example of creating a life estate, the dead person is able to control both what happens to their land immediately (their widow gets it) and what happens in the future when their widow dies.

It didn't end there. Let's say you were an army veteran and very religious. You could in your will give your widow a life estate, then your oldest direct male descendant an estate that lasts until he dies or until one of your direct descendants reaches the rank of Captain in the army or becomes a priest or nun in which case the first such direct descendant gets a life estate, with the remainder to their oldest child if they have one else to be sold at auction with the proceeds split equally among your living direct descendants.

Lord Montague could stick conditions in the estates he left in his will so that anyone who came into possession of the Montague lands after he died, no matter how long after, would lose their estate if they married a Capulet or descended from a Capulet.

In short if you owned a fee simple estate you could impose nearly arbitrary conditions on who could possess the land after you and what they could do with that could in theory restrict the land forever afterwards.

This makes it hard to put land to its economically best use. A seller cannot sell more than they have. I'm not going buy or lease your land to build my factory on, or build a housing development, or do anything that requires a significant investment if I have to worry that some third cousin of yours will marry a doctor and trigger some condition in the will of some ancestor of yours that died 200 years ago that gives them the land. Maybe my lawyers and genealogists can identify all people that might be beneficiaries of such un-triggered conditions and get them all to sell or lease me the land--but often even that won't be enough because some such people might have yet to be born.

Governments recognized that it is good for land to generally be freely sellable and buyable, and that the living should be able to fully use the land. And thus we got rules to limit the ability of dead people to place far reaching long lasting limits on what happened to their land after they died.

The rule against perpetuities strikes a quite reasonable balance between keeping land reasonably buyable and sellable with minimal restrictions on what the living can do with the land and giving people some say in how their land is inherited.

The classic statement of the rule is:

> No interest in land is good unless it must vest, if at all, not later than 21 years after some life in being at the time the interest was created.

This basically gives you the current generation plus 21 years of control over what happens to your land. It is enough that you can specify conditions involving grandchildren that do not yet exist, or even conditions on adult grandchildren that do not yet exist because if any such come into existence it will be within 21 years of the life of one of your existing children. But no more keeping your family forever Capulet free.

Details vary from jurisdiction to jurisdiction. Some places use longer than 21 years for example. Some state take a "wait and see" approach on vesting--if the interest has vested by the end of the measuring life it is valid even if there was a possibility it would not vest in time. Some places have a two pronged rule where an interest is valid if it satisfies that classic rule (or its "wait and see" variation if they use that) or it will either vest or terminate within N years of its creation. N is commonly 90, although Florida does N = 360.