Common law is the body of legal rules that judges have built through their decisions in individual cases, rather than rules a legislature wrote into a statute.1Legal Information Institute. Common Law When a court resolves a dispute, its reasoning becomes a reference point for the next case with similar facts, and over time those rulings settle into principles that govern everyday life. Useful examples of common law include negligence, strict liability for dangerous activities, the mailbox rule, the mirror image rule, adverse possession, prescriptive easements, common law marriage, respondeat superior, caveat emptor, and stare decisis. Each one below explains what the rule is and where it still shows up.
Negligence
Negligence is the foundation of most personal injury claims. A court asks whether the person who caused the harm acted with less care than an ordinary, reasonable person would have used in the same situation.2Legal Information Institute. Negligence The “reasonable person” test is deliberately objective. Being naturally careless or inexperienced is no defense; the standard is the same for everyone.
To win, the injured person generally has to prove four things: the other party owed a duty of care, they breached that duty, the breach actually caused the injury, and real damages resulted. Miss any one element and the claim fails.
Your own carelessness matters too. A few jurisdictions still follow the old contributory negligence rule, which bars recovery entirely if you were even slightly at fault. Most states use comparative negligence, which reduces your compensation in proportion to your share of the blame. In many of those states, being more than 50 percent responsible for your own injury means you recover nothing.
Strict Liability for Dangerous Activities
Some activities are so inherently risky that courts hold the person conducting them responsible for any harm that follows, no matter how careful they were.3Legal Information Institute. Ultrahazardous Activity American courts call these “abnormally dangerous” or “ultrahazardous” activities. Blasting with explosives is the classic example. Storing large quantities of toxic chemicals and keeping wild animals also qualify. The reasoning is that no amount of precaution makes these activities safe enough to push the risk onto innocent bystanders, so the operator pays if someone gets hurt.
The Mailbox Rule
The mailbox rule sets the exact moment a contract forms. Under this doctrine, an acceptance becomes effective the instant the accepting party sends it, not when the person who made the offer receives it.4Legal Information Institute. Mailbox Rule Drop your signed acceptance in a mailbox on Monday and the contract exists Monday, even if the letter doesn’t arrive until Wednesday.
Timing is where this matters. If someone tries to revoke the offer while your acceptance is in transit, the revocation comes too late because the contract already formed when you sent your response. The rule applies to faxes, emails, and other methods as long as the acceptance is irrevocable once sent. Parties can override it by writing into the offer that acceptance is only effective on receipt, but absent that kind of language, the default favors the person accepting.
One wrinkle: if you send a rejection first and then change your mind and send an acceptance, the outcome depends on which one arrives first. A late change of heart isn’t automatically protected.
The Mirror Image Rule
The mirror image rule requires an acceptance to match the offer exactly. If someone offers to sell you a car for $10,000 and you reply “I’ll take it for $9,500,” you haven’t accepted anything. Courts treat your response as a counteroffer that the original seller can accept or reject.5Legal Information Institute. Mirror Image Rule Even small changes to payment terms, delivery dates, or other conditions can stop a contract from forming.
The rule made sense for simple one-off deals and gets awkward when businesses trade lengthy purchase orders and invoices with boilerplate that never quite lines up. The Uniform Commercial Code addressed this for sales of goods by letting a contract form even when the acceptance includes additional or different terms, as long as the response is a definite expression of acceptance. Between merchants, those extra terms can even become part of the contract unless they materially change the deal. The traditional mirror image rule still applies to service contracts, real estate deals, and other agreements outside the UCC’s scope.
Adverse Possession
Adverse possession lets someone gain legal ownership of land they don’t hold title to, if they meet a strict set of conditions over a long period. The possession has to be continuous, open and visible to anyone who looks, and hostile to the actual owner’s rights.6Legal Information Institute. Adverse Possession “Hostile” doesn’t mean aggressive. It just means the occupant is using the land without permission and as if they owned it.
The required time period varies by state, typically running from about 5 to 20 years. Some states shorten the clock when the occupant holds a defective deed or other document that looks like it gives them title but is legally insufficient. This is known as “color of title,” and it often cuts the waiting period roughly in half because the occupant at least had a good-faith basis for believing they owned the property.
The doctrine exists for practical reasons. It encourages productive use of land and clears up ownership disputes that have lingered for decades. If an owner ignores someone openly occupying their property for the statutory period, the law eventually sides with the person putting the land to use.
Prescriptive Easements
A prescriptive easement works on similar logic to adverse possession, but it grants only a right to use someone else’s land, not ownership.7Legal Information Institute. Prescriptive Easement The classic scenario is a neighbor who crosses your property using the same path for years, openly and without your permission. If that use continues long enough to satisfy the period set by state law, the neighbor earns a legal right to keep using the path even if you later try to block it.
The requirements mirror adverse possession: open, continuous, and without the landowner’s consent. One difference is that prescriptive easements only cover affirmative uses, where someone is actively doing something on the land. You can’t acquire a prescriptive right to stop your neighbor from doing something on their own property.
Common Law Marriage
Common law marriage lets two people be legally married without a license or ceremony, but only in the roughly ten states and the District of Columbia that still recognize it. Requirements vary, but courts generally look for three things: both people have the legal capacity to marry, they share a present intent to be married to each other right now, and they hold themselves out publicly as a married couple.8U.S. Department of Labor. Common-Law Marriage Handbook
Legal capacity means both people are old enough, mentally competent, and not already married to someone else. Present intent means agreeing that “we are married,” not “we plan to marry someday.” Holding out means acting like a married couple in the community: using the same last name, introducing each other as spouses, filing joint tax returns, or sharing finances in ways consistent with marriage.8U.S. Department of Labor. Common-Law Marriage Handbook Simply living together for a long time, without the mutual intent and public presentation, isn’t enough.
A common law marriage formed in a state that recognizes it generally carries the same legal weight everywhere else. The IRS treats a valid common law marriage the same as a ceremonial one, so couples can file joint federal tax returns regardless of where they later move.9Internal Revenue Service. Revenue Ruling 2013-17 Under the Family and Medical Leave Act, “spouse” includes a common law husband or wife from any state that recognizes such marriages.10U.S. Department of Labor. Fact Sheet 28L – Leave Under the Family and Medical Leave Act The Social Security Administration also pays survivor and dependent benefits to common law spouses who can document the marriage, usually through sworn statements from each partner and blood relatives.
Respondeat Superior
Respondeat superior makes employers legally responsible for harm caused by their employees while they are doing their jobs.11Legal Information Institute. Respondeat Superior If a delivery driver runs a red light and injures a pedestrian during a delivery route, the injured person can sue both the driver and the employer. The doctrine recognizes that businesses profit from their workers’ activities and are better positioned to absorb the financial risk of accidents.
The key question is always whether the employee was acting within the scope of their employment. A driver who stops briefly for coffee on the way to a delivery is still within scope. A driver who abandons the route entirely to visit a friend two towns over has gone on what courts call a “frolic,” and the employer generally isn’t liable for anything that happens during that personal side trip. Where the line falls is fact-intensive and comes up in litigation constantly.
The doctrine does not extend to independent contractors.11Legal Information Institute. Respondeat Superior Courts use a multi-factor test to sort employees from contractors, looking at things like how much control the hiring party exercises over the details of the work, whose tools the worker uses, how they’re paid, and whether the work is part of the hiring party’s regular business. The more control the hiring party has, the more likely the doctrine applies.
Caveat Emptor and Modern Warranty Law
Caveat emptor, meaning “let the buyer beware,” is one of the oldest commercial principles in common law. Buyers were expected to inspect goods themselves before purchasing. If something turned out to be defective, that was the buyer’s problem, not the seller’s, unless the seller had made a specific promise.
Modern law has substantially narrowed this rule. The Uniform Commercial Code, adopted in some form by every state, automatically attaches an implied warranty of merchantability to any sale of goods by a merchant.12Legal Information Institute. UCC 2-314 Implied Warranty – Merchantability That warranty requires goods to be fit for their ordinary purpose, pass without objection in the trade, and conform to any promises on the label. A toaster that catches fire the first time you use it breaches this warranty whether or not you inspected it at the store.
Federal law adds another layer. The Magnuson-Moss Warranty Act requires any written warranty on a consumer product to be clearly labeled as either “Full” or “Limited,” and it prohibits sellers from disclaiming implied warranties when they offer a written warranty.13Federal Trade Commission. Businesspersons Guide to Federal Warranty Law Caveat emptor still surfaces in some real estate transactions and private sales between non-merchants, but for most consumer purchases, the old rule has been replaced by protections that put the risk of hidden defects back on the seller.
Stare Decisis
Stare decisis is the principle that holds the whole common law system together. It directs courts to follow the reasoning of earlier decisions when facing cases with similar facts.14Legal Information Institute. Stare Decisis Without it, every lawsuit would start from scratch and outcomes could swing based on which judge happened to hear the case.
The doctrine operates on two levels. Vertical stare decisis means lower courts must follow the rulings of higher courts in the same jurisdiction. A federal district court is bound by the decisions of its circuit court of appeals, which in turn is bound by the Supreme Court.15Constitution Annotated. Historical Background on Stare Decisis Doctrine Horizontal stare decisis means a court generally follows its own prior decisions, though this constraint is weaker. The Supreme Court, for example, can overrule its own precedent when it concludes an earlier decision was wrong, though it does so rarely.
Stare decisis is what lets lawyers predict how a court will rule and, more importantly, what lets ordinary people plan around settled legal rules. Every other doctrine on this list exists in its current form because courts followed and refined earlier decisions over decades, sometimes centuries. Judges still distinguish new facts from old ones, carve out exceptions, and occasionally overrule outdated reasoning, but the baseline expectation is consistency, and that expectation is what gives common law its force.