Definition
LAT is an abbreviation with several distinct meanings depending on legal and regulatory context. In maritime, admiralty, and coastal property law, it most commonly stands for:
1. LOWEST ASTRONOMICAL TIDE. The lowest level that tidal waters can be predicted to reach under average meteorological conditions and any combination of astronomical factors. LAT is used as a tidal datum — a reference baseline for measuring water depths on nautical charts and for defining coastal boundaries, including the limits of territorial waters, port structures, and foreshore rights.
2. LIVING APART TOGETHER. In family law and social welfare contexts, LAT refers to a recognized domestic relationship in which two persons are in an ongoing committed partnership but maintain separate primary residences. Legal significance arises in jurisdictions where LAT relationships may affect entitlement to spousal benefits, government assistance, inheritance rights, or claims under cohabitation statutes.
The maritime meaning (LAT as Lowest Astronomical Tide) is the more frequently litigated of the two and the more significant for legal research in property, admiralty, and environmental law.
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Common Language
Modern common usage (Wiktionary): LAT appears as an initialism with at least four distinct common meanings: lowest astronomical tide, laboratory animal technician, large air tanker, and living apart together.
Historical common usage (Webster's 1913): The entry records "lat" as an obsolete verb meaning "to let; to allow," attributed to Chaucer — a survival from Old and Middle English with no continuing legal relevance.
The gap between common and legal usage is material in two directions. First, the Webster's 1913 verbal meaning is entirely obsolete and will mislead researchers encountering "lat" in medieval or early modern legal texts, where it may appear as a term of permission rather than a modern abbreviation. Second, among the current Wiktionary meanings, only the tidal datum and living-apart-together senses carry genuine legal weight; laboratory animal technician and large air tanker are technical or administrative designations that rarely generate legal controversy of their own.
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Common Confusion
LAT (Lowest Astronomical Tide) is frequently confused with related tidal datums, particularly MLLW (Mean Lower Low Water) and MLW (Mean Low Water). These are not interchangeable. LAT represents the theoretical minimum — the lowest the tide can predictably fall — while MLLW and MLW are averages calculated over tidal cycles. Different jurisdictions and different legal instruments (admiralty charts, property deeds, environmental permits) may reference different datums, and using the wrong one can produce materially different boundary lines or depth calculations.
In family law, LAT (Living Apart Together) is sometimes conflated with legal separation or with common-law cohabitation. A LAT couple has not necessarily undergone any formal legal process, and their relationship may or may not meet the threshold for statutory recognition as a domestic partnership or de facto relationship depending on jurisdiction.
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Why It Matters in Research
For researchers working in coastal property, admiralty, or environmental law, the distinction between LAT and other tidal datums is not merely technical — it determines where the legal boundary of the foreshore lies, which in turn affects ownership claims, public access rights, navigation easements, and regulatory jurisdiction. International hydrographic standards favor LAT as the chart datum of reference, but domestic legal instruments in the United States have historically relied on MLLW or the mean low water line. Researchers must check which datum is operative in the specific instrument, statute, or regulation under review.
For family law researchers, the LAT relationship is a relatively recent category in legal discourse, gaining prominence as changing social patterns produced committed couples who deliberately maintain separate households. It is underrepresented in older treatises and case law. Researchers should look to statutory definitions of "cohabitation," "domestic partner," and "de facto relationship" to determine whether a LAT arrangement triggers legal consequences in a given jurisdiction — the answer varies widely.
Historical sources, including Burrill, use neither of these modern abbreviations. The Burrill entry reproduced here (under LARGURE) concerns entirely unrelated material — French-law terms for breadth and length of commodities — and has no connection to the modern abbreviation LAT. Researchers consulting Burrill for this term will not find relevant guidance.
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Historical Dictionary Support
Burrill's Law Dictionary does not contain an entry for LAT as a modern abbreviation. The source material supplied reflects adjacent entries dealing with Norman-French legal vocabulary (largure, large) concerning measurements and an ancient court of East Kent — contextually unrelated to any current use of LAT. This is an important negative finding: the abbreviation LAT in its maritime tidal datum sense is a product of modern hydrographic standardization, not historical common law, and pre-twentieth-century legal dictionaries will not illuminate it.
The obsolete verbal sense recorded in Webster's 1913 — "to let; to allow," from Chaucer — may occasionally surface in transcriptions of medieval English legal documents. Researchers in legal history should be aware of this usage to avoid misreading archaic text, but it carries no operative legal significance in modern practice.
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Jurisdictional Note
The operative tidal datum for legal boundary purposes varies by jurisdiction. In the United States, federal and state property law has generally used the mean low water line rather than LAT; international maritime law and hydrographic charting increasingly uses LAT per International Hydrographic Organization standards. In family law, recognition of LAT relationships as legally cognizable is largely a matter of provincial or state statute, with Canadian provinces and several Commonwealth jurisdictions further developed in this area than most U.S. states.
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