Definition
Interest upon interest is the charging or accrual of interest on amounts that are themselves interest — that is, the addition of unpaid interest to the principal balance so that the resulting sum becomes the new base upon which further interest is calculated. It is also called compound interest. In its most straightforward form: if a borrower owes $1,000 in principal and $100 in accrued interest, and that $100 is capitalized into the debt, the borrower then owes interest on $1,100 rather than on $1,000 alone.
The term appears most frequently in older legal sources as a term of reproach or legal limitation — courts historically distinguished simple interest (interest on principal only) from interest upon interest (interest on accumulated interest) as a matter of equity and public policy. The distinction carried significant practical consequences: a creditor who sought to recover interest upon interest might be denied recovery altogether if no lawful authority — statute, express agreement, or mercantile custom — supported the claim.
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Common Language
Modern common usage (Wiktionary): "Compound interest" is the standard modern term; "interest upon interest" does not appear as a standalone Wiktionary entry, though the concept is described under compound interest as interest calculated on both the initial principal and the accumulated interest from prior periods.
Historical common usage (Webster's 1913): Webster's 1913 does not carry a standalone entry for "interest upon interest," but defines compound interest as "interest, not only on the original principal, but also on the interest which has accrued from time to time."
The legal term "interest upon interest" is the older formulation that legal dictionaries and courts preferred well into the nineteenth century. Researchers encountering the phrase in historical pleadings, judicial opinions, or equity proceedings should recognize it as synonymous with compound interest but carrying its own body of doctrine — particularly regarding when such interest was permissible and when it was void as against public policy. The modern financial term "compound interest" has largely displaced it in transactional and statutory contexts.
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Common Confusion
INTEREST UPON INTEREST vs. COMPOUND INTEREST: These terms describe the same mathematical phenomenon but carry different doctrinal freight in historical legal sources. "Compound interest" is the neutral, modern financial term freely used in contracts and statutes. "Interest upon interest" in older legal writing often signals a doctrinal question — whether the practice was lawful at all — rather than merely a description of the calculation method. Courts and treatise writers who used "interest upon interest" frequently did so in the context of denying or limiting recovery. Researchers should not assume that sources approving compound interest in modern contracts are resolving the same question addressed in historical cases discussing interest upon interest.
INTEREST UPON INTEREST vs. USURY: Though distinct, the two concepts intersected in historical doctrine. Charging interest upon interest was sometimes treated as presumptively usurious, or as a device to evade usury ceilings. They are not synonymous: usury concerns an unlawfully excessive rate; interest upon interest concerns the base on which interest is calculated. A loan could be usurious without compounding, and compounding could occur within lawful rate limits.
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Why It Matters in Research
The practical importance of this term in legal research is almost entirely historical. Modern contract and commercial law generally permits compound interest when the parties expressly agree to it, and statutory frameworks (particularly in tax, consumer lending, and judgment-interest contexts) specify compounding rules directly. The doctrinal action surrounding "interest upon interest" belongs largely to the nineteenth century and earlier.
Several traps await researchers working in historical sources:
First, the rule against interest upon interest was not uniform. English equity courts were generally hostile to it absent agreement or mercantile custom. American courts diverged, with some states permitting it by statute and others following the equity rule. A case denying interest upon interest in one jurisdiction does not state a general rule.
Second, an important exception ran throughout the case law: rests. In accounting between parties with long-running financial relationships — particularly in mortgage, partnership, and trust contexts — courts would sometimes strike periodic "rests," treating accumulated interest as added to principal at defined intervals, then allowing interest on the new total. This was a judicially sanctioned form of interest upon interest, and historical sources treating rests as permissible are not in conflict with the general rule against interest upon interest; they represent a recognized exception.
Third, in the context of judgments, older courts were reluctant to compound interest on a judgment — interest running on the judgment amount was generally simple interest only. Modern jurisdictions vary on this, and some federal statutory schemes (including tax overpayment contexts) address compounding directly.
Fourth, in trust and fiduciary accounting, the question of whether a defaulting trustee owed interest upon interest on withheld funds was genuinely contested. Some courts imposed it as a punitive measure against a faithless fiduciary; others refused on the ground that no agreement supported it.
Researchers using Rapalje & Lawrence should treat its entry as a reliable statement of the late-nineteenth-century American legal understanding, but should check whether the relevant jurisdiction had a governing statute before relying on the common-law baseline.
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Historical Dictionary Support
Rapalje & Lawrence define interest upon interest straightforwardly as compound interest — interest charged on accrued interest that has been added to the principal — and note the general common-law rule that it is not allowable without special agreement, statute, or recognized mercantile usage.
This framing reflects the dominant nineteenth-century American equity position. Historical dictionaries generally agree on the definition but vary in how emphatically they state the prohibition. Some sources treat the rule as nearly absolute absent express agreement; others acknowledge that commercial practice — particularly in banking and mercantile accounts where periodic rests were customary — had carved out significant exceptions.
What historical dictionaries often miss or understate: (1) the growing divergence between equity-court doctrine and commercial reality by the late 1800s, as banking instruments routinely provided for compounding; (2) the developing statutory landscape, which increasingly displaced the common-law rule; and (3) the fiduciary context, where courts fashioned a separate body of doctrine around compound interest as a remedy against defaulting trustees that did not fit neatly within the standard contract-based analysis.
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Jurisdictional Note
American jurisdictions diverged significantly on this doctrine by the mid-nineteenth century. Several states enacted statutes expressly permitting interest upon interest when contracted for, effectively abrogating the equity rule. Others maintained the restriction. The federal courts generally followed the law of the state in which they sat for questions of interest on private obligations, meaning jurisdiction is an essential variable in any historical research involving this term.
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Encyclopedia Cross-Reference
The Law Mind Tax Encyclopedia: Interest on Overpayments — relevant for modern statutory compound-interest treatment in federal tax contexts, where compounding rules are governed by the Internal Revenue Code rather than common-law doctrine.
The Law Mind Trusts, Estates & Probate Encyclopedia: Classification of Future Interests — tangentially relevant for researchers encountering interest-upon-interest questions in trust accounting and fiduciary liability contexts.
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