Definition
In historical English law, tau refers to a cross, a meaning derived from the Greek letter tau (τ), which takes the shape of a T-shaped or Latin cross. The term appears primarily in the compound phrase TAURI LIBERI LIBERTAS, where it anchors a specialized manorial law concept: the right of tenants within a manor, liberty, or similar jurisdiction to have access to a common bull (taurus liber, literally "free bull") for the breeding of their cattle. The bull was considered "free" in the sense of being available to all qualifying tenants within the territory, not reserved to any single holder.
The bare word tau, standing alone in legal materials, is rarely encountered except as a component of this phrase or as a descriptive marker for a cross-shaped boundary marker or seal impression.
Common Language
Modern common usage (Wiktionary): Tau is the nineteenth letter of the Greek alphabet (τ, Τ). In modern English, it appears in scientific and mathematical contexts to denote various constants and ratios.
Historical common usage (Webster's 1913): Webster's identifies tau primarily by reference to the T-shaped tau cross and notes the name applied to the common American toadfish on account of a tau-shaped marking.
The gap between common and legal usage here is narrow but specific: the legal tradition appropriates the letter's shape — the cross — rather than the letter itself, and uses it as a shorthand for documentary or boundary crosses in early records. A researcher encountering "tau" in a manorial or ecclesiastical document should consider whether it denotes a literal cross-mark (as a signature substitute or land marker) rather than any substantive legal concept.
Common Confusion
TAU vs. CROSS (legal mark): In early medieval and post-Conquest records, a tau mark or cross served as a signature substitute for illiterate parties. Researchers sometimes conflate this evidentiary function — a cross as authentication — with the manorial law concept of TAURI LIBERI LIBERTAS, which uses tau etymologically but concerns livestock rights entirely. The two usages share the same root but operate in wholly different legal contexts.
TAU vs. TAURUS: The legal compound TAURI LIBERI LIBERTAS derives from taurus (bull), not from tau (cross). Rapalje & Lawrence's entry links them by proximity, which can mislead a reader into thinking tau itself means bull. It does not. Tau means cross; the manorial phrase is a separate composite term.
Why It Matters in Research
Researchers are most likely to encounter tau in two distinct situations in the Law Mind corpus, and conflating them will produce errors.
First, in manorial court records, extents, and custumals, TAURI LIBERI LIBERTAS describes a specific communal right. When researching grazing rights, common pasture, or the obligations of lords of the manor regarding livestock, this phrase signals a recognized entitlement enforceable in manorial courts. The right is analogous to other appurtenant benefits running with copyhold or customary tenure.
Second, in medieval charters, deeds, and probate instruments, a tau cross appears as an authenticating mark — functionally equivalent to a signature — made by a party unable to write. This use is important for document authentication and questions of whether an instrument was duly executed. Researchers working with pre-modern instruments should understand that a cross mark carries legal weight as a deliberate act of execution, not mere decoration.
Jurisdictional trap: This term is specific to English manorial law and does not translate to American, Scottish, or continental European jurisdictions without substantial qualification. American property researchers will rarely encounter tau except in historical materials concerning land grants derived from English colonial practice.
The term is sparse in the Law Mind corpus because it functions more as a signpost within compound phrases than as a freestanding legal concept. Searches for tau alone will underperform; searches for TAURI LIBERI LIBERTAS or cross-mark authentication will retrieve the relevant materials.
Historical Dictionary Support
Rapalje & Lawrence is the sole source dictionary for this entry, and the coverage is brief: tau is defined as "a cross" with a citation to Selden, followed immediately by the compound phrase TAURI LIBERI LIBERTAS and its definition. The entry's compression is characteristic of Rapalje & Lawrence's treatment of Latin and quasi-Latin terms that function primarily as components of longer legal phrases.
The Selden citation is notable. John Selden's works — particularly Titles of Honor and his notes to Fortescue — engage extensively with heraldic and documentary cross-marks, lending authority to the cross-as-tau identification in English legal usage. Researchers needing depth on the cross as a legal mark should consult Selden directly rather than relying on the dictionary entry alone.
No other historical law dictionaries in the standard shelf (Bouvier, Black, Burrill, Tomlin, Jacob) carry a substantive entry for tau, which suggests the term was understood as sufficiently covered by entries for cross or by the manorial law entries under bull or common rights. This gap means that Law Mind corpus searches relying solely on dictionary headwords will miss relevant doctrine that is indexed under adjacent terms.
Jurisdictional Note
Tau as a legal term belongs to the English common law tradition, specifically to manorial law and to medieval documentary practice. It has no recognized equivalent in American statutory or common law. Researchers working on English land history, ecclesiastical records, or medieval charters will encounter it; those working in modern Anglo-American law will not.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia — Manorial Rights
See Law Mind Encyclopedia — Common Lands
See Law Mind Encyclopedia — Authentication of Instruments (historical)