The Verification Venue · pointed at a rule of possibility, not probability
Twenty-One Years Past the Last Friend
A popular summary says the Rule Against Perpetuities gives a gift twenty-one years. It gives nothing of the sort. An interest is void unless it must vest, if at all, within twenty-one years of the death of some life in being at the creation of the interest, and the test is possibility, not probability: one far-fetched birthday, of a child not yet conceived, kills a gift that would almost certainly vest.
Below is a bench of five conveyances. Choose one and the page runs an exact decision procedure over it: a search of the space of possible births and deaths for a single scenario in which the gift vests too late. Find one and the gift is dead, and the page hands you the scenario, year by year. Find none and the gift is safe. Drag the age condition and watch the verdict turn at twenty-one.
Five conveyances, one rule
These are teaching constructs, not forms for use. The page models the classical common-law rule and refuses to speak for any jurisdiction's current law: most US states have modified or abolished the rule by statute, and the details vary and change. ↓
Verdict on the headline interest
Latest possible vesting (computed, not measured)
the rule allows 21 years after the last life in being dies
Drag to 21 and the class at 25 becomes safe; drag to 22 and it dies again. The boundary is exact, because a child born as the last life in being dies turns N exactly N years later.
Start with the phrase everyone garbles. A life in being is a natural person alive at the moment the instrument takes effect; the classical rule adds a child in utero, if later born alive. It does not mean the people the document happens to name, and it does not mean the living in general. The period runs from the death of the last survivor of all the lives in being, and nobody chooses which death that is: the adversary does, because the rule asks what could happen, not what probably will.
The page's procedure is that adversary made mechanical. For each future interest it solves, over integer years and with exact arithmetic, for the latest vesting any allowed world permits, then compares with the window. When the interest is void it does not merely say so: it constructs the offending world, the specific sequence of births and deaths under which vesting comes too late.
Now read the witness for the class at 25. Anna dies in year 1 leaving a child born that same year; the classical rule presumes any living person fertile until death, so the world is allowed. The child turns 25 in year 26, which is 25 years after the only life in being died, and 25 is not 21. The gift is void, even if Anna already has adult children whose shares looked like sure things. Notice what protects the children already born: a child in being who lives to vest anchors the period with its own death. The killer is always the child who is not yet anybody's life in being. Drag the condition down to 21 and the door shuts in time; at 22 it swings open again.
Fiction I · the fertile octogenarian
Take the rule's own logic seriously and it presumes that any living person may leave a child, however old. The legal literature's name for this is the fertile octogenarian. Grace is 87. Under the classical presumption she can die leaving an infant, and a class gift at 30 dies with her, exactly as the class at 25 did. Nothing here is a currently binding presumption anywhere; it is the classical rule's own assumption, shown operating. Cap fertility at 60 (a modelling choice of this page, not a statute) and the same gift lives, provided Grace is old enough that no after-born child is possible.
Under the modern cap, drag downward and find the age at which an after-born child becomes possible and the gift dies.
Grace's age 20 to 95, one glyph each: · the gift is valid, ✗ the gift is void. Hover or focus a glyph for the age. Computed live, one solver run per age.
Fiction II · the unborn widow
To Henri for life, then to Henri's widow for life, then to the harbour mission. The widow Henri actually has may be a life in being. But Henri might marry, in his last year, a woman born the week before: she is his widow, and she was nobody in being when the deed took effect. Her death can come arbitrarily late, and the mission's gift dies of it, while the widow's own life estate is perfectly safe. Some courts, unwilling to kill such gifts, presumed the widow was the grantor's current wife: a life in being, and the gift lives. That presumption is history's escape hatch, not current law; both branches below are the classical rule's own logic, with the hatch shown.
The check: every number recomputed in front of you
Six conveyances, decided two ways. The solver column is the constraint procedure running above. The enumeration column is a structurally independent route: an exhaustive pass over every world on a bounded 34-year grid, births, deaths, marriages and all. The settled column is not a measurement; it is the classical rule's settled answer for each conveyance as stated in standard treatments, and each conveyance is written out in full so you can check the mapping yourself.
| conveyance | settled (doctrine) | solver | enumeration | agree |
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·
What is exact: the decision procedure is an exhaustive adversary over the modelled clause language in integer arithmetic, and the enumeration re-decides every row by brute force; their agreement is printed above, live. What would make this page wrong: a settled answer misstated, a disagreement between the routes, or a witness that violates the model's own constraints. Run it yourself: node research/twenty-one-years-past-the-last-friend/verify-twenty-one-years-past-the-last-friend.mjs.
What's exact here, what's idealised, and what the page refuses to claim
Exactly true, inside the model. The decision procedure is exact over the modelled clause language: it searches the space of allowed worlds with integer constraint solving, and a single witness settles invalidity because the rule is a rule of possibility, while exhausting the search settles validity. The enumeration route re-derives every verdict by brute force on a grid wide enough for every witness these conveyances can produce (the widest excess any of them can generate is bounded by the largest age condition, 40, well inside the 34-year grid once the anchor lives are accounted for).
Idealised. Time runs in whole years from the deed taking effect at year 0. Gestation is not modelled: a birth in year 0 counts as a child in being at creation, a birth in year 1 or later as conceived after creation. Marriage is modelled as "the widow was born at least a year before Henri died". The classical fertility presumption (any living person may leave a child until death) is the rule's own assumption; the modern cap of 60 is this page's modelling choice, not a statute. The clause language admits one class gift, one widow clause, and the vesting patterns shown; anything else is rejected as outside the model rather than guessed at. Vesting for a class is judged member by member at the moment each member satisfies the age condition; class-closing doctrines, powers of appointment, divestment and the identity-of-heirs questions are out of scope. "Must vest, if at all" is modelled literally: a world in which the gift never vests offends nothing.
Representative, not universal, and not current law. The settled column records the classical common-law rule's answers as presented in standard treatments; those answers are doctrine, not measurements, and this page checks them against its own two routes rather than against any survey. Most US states have modified the rule (wait-and-see statutes, the Uniform Statutory Rule Against Perpetuities) or abolished it outright, and other jurisdictions have replaced it by their own statutes; the details vary by state and change over time. This page states no jurisdiction's current law and ships no jurisdiction selector, because checking one would require a fifty-state survey this page cannot run. The fertile octogenarian and the unborn widow are features of the classical rule and of the cases that made its problems visible; neither is offered as a binding presumption anywhere today.