Definition
A diagram or table, arranged in the shape of a tree with branching lines, that maps the blood relationships within a family. The term is Latin for "tree of consanguinity" or "tree of blood relation." The trunk or root represents a common ancestor, and the branches extending outward represent successive generations of descendants or collateral relatives. The visual structure was used by civil and canon lawyers to determine degrees of kinship — that is, how closely two individuals are related by blood — which had direct legal consequences for inheritance, marriage eligibility, and the capacity to testify or hold certain offices.
The arbor consanguinitatis is closely associated with the arbor civilis, the civil-law version of the kinship diagram used in Roman and civilian legal traditions to calculate degrees of relationship for succession purposes. Canon law developed a parallel diagram, sometimes called the arbor canonica, applying a different method of counting degrees — a distinction that produced significant practical divergence between civil and ecclesiastical determinations of prohibited marriages and inheritance rights.
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Common Confusion
The arbor consanguinitatis is sometimes conflated with the arbor affinitatis, a related diagram used by canonists to map relationships by marriage rather than blood. The two trees operate on the same diagrammatic principle but govern different legal questions: consanguinity concerns blood kin, while affinity concerns kin acquired through the marriage bond. Historical sources, particularly in ecclesiastical law, address both trees together, and a researcher encountering one will frequently need to consult the other. A third related diagram, the arbor civilis, tracks the same blood relationships as the consanguinitatis tree but applies the civil-law method of counting degrees, which differs from the canonical method — a distinction with real consequences when inheritance disputes crossed jurisdictional lines between ecclesiastical and secular courts.
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Why It Matters in Research
The arbor consanguinitatis is not merely a genealogical curiosity. It was a functional legal instrument with direct operational consequences in at least three areas of historical law:
First, inheritance. Civil law counted degrees of kinship by the number of generational steps between two individuals through their common ancestor. The degree determined priority among heirs in intestate succession. A researcher working with estate disputes in civil-law jurisdictions — or in English common law cases influenced by civilian doctrine — will encounter references to "degrees" of relationship that only make sense against the background of how the arbor was used to count them.
Second, marriage prohibition. Canon law prohibited marriage within certain degrees of consanguinity and affinity. The canonical method of counting degrees differed from the civil method: canonists counted to the common ancestor and stopped there, while civilians counted all steps up and back down. This means a marriage that was prohibited under canon law might not have been classified in the same degree under civil law. Researchers working with matrimonial causes, annulment proceedings, or dispensation records must be alert to which counting system the source is applying.
Third, testimonial competency and office-holding. Some legal systems restricted the ability of close relatives to testify against one another or to hold simultaneous offices. The arbor was the reference tool for settling those questions.
For corpus researchers: references to the arbor consanguinitatis in historical English legal sources typically appear in the context of common-law borrowing from civilian doctrine, particularly in discussions of inheritance and the ecclesiastical courts' jurisdiction over marriage. Hale's History of the Common Law (referenced in both Black's editions) is a useful entry point for understanding how English common lawyers engaged with civilian genealogical tables. The maxim appended in Black's — Arbor dum crescit, lignum dum crescere nescit ("a tree while it grows, wood when it ceases to grow") — is a separate property-law principle about the legal status of timber and should not be read as related to the consanguinity table; its appearance in the same entry is an artifact of alphabetical proximity in the source dictionaries.
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Historical Dictionary Support
Both the first and second editions of Black's Law Dictionary provide essentially identical entries, defining the arbor consanguinitatis as a table formed in the shape of a tree showing family genealogy, with a cross-reference to the arbor civilis of the civilians and canonists and a citation to Hale's History of the Common Law. The entries are brief and descriptive rather than analytical.
Neither edition explains the practical legal function of the diagram in any detail, nor do they distinguish between the civil and canonical methods of counting degrees — a gap that matters considerably for research into matrimonial or succession disputes. The appended maxim about trees and timber (Arbor dum crescit, lignum dum crescere nescit) appears in both editions without explanation of its connection, if any, to the main entry; it is best understood as a separate Latin legal maxim included under the same head word by convention.
The references to Cro. Jac. 166 and Hob. 77b cited alongside the timber maxim are to Croke's Jacobean reports and Hobart's reports respectively, and relate to the property-law question of when standing timber becomes legally classified as wood — not to genealogical tables.
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Jurisdictional Note
The arbor consanguinitatis as a legal instrument was primarily operative in civil-law and canon-law jurisdictions. Its influence in English common law was indirect, mediated through ecclesiastical court practice and civilian-trained lawyers. Researchers working in Scottish law, which retained a stronger civilian tradition, will encounter the diagram's logic more directly in succession materials than in English common-law sources.
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