Definition
Perdurable means lasting long or forever. In property law, the term describes the quality of permanence or durability attached to an estate or a right — specifically, whether that estate or right will endure without limitation or diminishment over time. An estate is perdurable when nothing in its constitution causes it to terminate or be cut short; it continues indefinitely unless some external legal event extinguishes it.
The term appears most frequently in two contexts:
1. Comparison of estates: A fee simple absolute is the most perdurable estate because no condition, limitation, or contingency attaches to it. A defeasible fee — such as a fee simple on condition — may be just as large in scope as a fee simple absolute, but it is not equally perdurable because a triggering event can terminate it.
2. Unity of seisin and extinguishment of rights: Perdurability bears on whether a right (such as an easement or profit) is extinguished when unity of seisin merges ownership of the dominant and servient estates. Extinguishment by unity of seisin requires that both the right and the land out of which it issues be held by the same person in estates of equal quality and perdurability. If the estates are not equally perdurable — for instance, one is fee simple absolute and the other is a fee on condition — unity of seisin does not fully extinguish the right, because the estates may separate again.
Common Language
Modern common usage (Wiktionary): Very durable; long-lasting.
Historical common usage (Webster's 1913): Very durable; lasting; continuing long. Marked as archaic.
The common meaning tracks the legal meaning more closely than is typical, but the gap is meaningful in practice. In ordinary use, "perdurable" is simply a strong synonym for "lasting." In law, it carries technical weight: it describes a quality of an estate that determines whether unity of seisin operates to extinguish servitudes, and it provides the conceptual basis for distinguishing the practical scope of an estate from its legal endurance. The legal use is not merely descriptive — it has consequences for the survival or destruction of property rights.
Why It Matters in Research
Perdurable is a term of art that researchers encounter almost exclusively in older property law sources — treatises on real property, early common law texts, and cases dealing with easements, profits, and the doctrine of unity of seisin. It rarely appears in modern statutory language or recent case law without reference back to its classical property law context.
The key research trap: because perdurable sounds like a general intensifier ("very lasting"), its technical role in the unity of seisin analysis is easily missed. When you see the term in a historical source, look for what it is being contrasted with — the contrast between a fee simple absolute and a defeasible fee, or between two estates of different quality, is almost always the operative point.
Researchers working on extinguishment of easements or profits by merger should treat perdurability as a threshold question: courts applying the common law rule required equal perdurability before finding extinguishment. A historical source that uses the term without explanation is almost certainly drawing on Coke, Littleton, or one of the standard English real property treatises.
The term also appears in discussions of disseisor's rights — where a disseisor holds as large an estate as the true owner in terms of quantum, but not in terms of perdurability, because the disseisee retains the right to re-enter.
Modern property law has largely replaced this vocabulary with more direct language about defeasibility, merger, and extinguishment, so researchers should be prepared to translate.
Historical Dictionary Support
Both editions of Black's Law Dictionary provide nearly identical treatment, and both are truncated — the second edition cuts off mid-sentence at the unity of seisin discussion. This is a meaningful gap: the full doctrine requires understanding that unity of seisin extinguishes a right only when both the right and the land are held in estates of equal degree and perdurability. Neither edition fully states this condition.
Both editions agree on the core meaning: perdurable means lasting long or forever, and the term operates as a comparative quality of estates. Webster's 1913 confirms the word was already archaic in general English by the early twentieth century, which explains its confinement to technical property law usage by the time Black's was compiled.
No significant divergence exists between the two Black's editions on this term. The truncation in both entries suggests the compilers treated the term as a cross-reference point rather than a freestanding doctrine — researchers should pursue the unity of seisin doctrine directly for the full analysis.
Jurisdictional Note
Perdurability as a formal term of art is essentially a common law property concept inherited from English law. American jurisdictions that have modernized their property codes or adopted the Restatement (Third) of Property approach to servitudes do not use the term, though the underlying concept — whether merger extinguishes a servitude when the unified estates are not of equal quality — survives in modified form.