To infringe the patent you must do something specific, and it is narrower than most people assume.
Every element of a claim must be present. Not most of them, not the important ones — every one. A product missing a single claimed element does not literally infringe, however similar it looks.
Similarity is not the test. Two products can look identical and one infringe while the other does not, because the claim recites a detail only one of them has.
And intent is irrelevant. Direct infringement is strict liability. You can infringe a patent you have never heard of, held by a company you have never heard of.
The acts that infringe
Under 35 U.S.C. 271(a), five acts, in the US, during the term, without authorisation.
| Act | Notes |
|---|---|
| Make | Manufacturing, including for internal use |
| Use | Operating it, including testing |
| Offer to sell | The offer itself, before any sale |
| Sell | The transaction |
| Import | Bringing it into the US |
Offering to sell is a separate act. A quotation or a listing can infringe before anything is manufactured or delivered.
Using infringes even without selling. A company that builds one patented machine for its own factory infringes by using it.
All five are territorial. A US patent covers acts in the US. Making a product abroad and keeping it abroad does not infringe it, which is why rights are filed country by country.
The all-elements rule
The single most important concept, and the one that decides most analyses.
| Claim recites | Accused product has | Literal infringement? |
|---|---|---|
| A, B, C | A, B, C | Yes |
| A, B, C | A, B, C, D | Yes — extra features do not avoid it |
| A, B, C | A, B | No |
| A, B, C | A, B, and something like C | Maybe — equivalents |
Adding features never avoids infringement. A product containing everything claimed plus improvements still infringes. This surprises people who assume their version is different enough.
Omitting one element defeats it entirely. Which is why claim drafting matters so much: every word an applicant adds to a claim is a word a competitor can design around.
Compare against the claims, not the patent. The title, abstract and description have no effect on scope.
Independent and dependent claims
| Claim type | Scope | If you avoid it |
|---|---|---|
| Independent | Broadest, stands alone | You avoid everything depending on it |
| Dependent | Narrower, incorporates its parent | Only relevant if the parent is infringed |
Start with the independent claims. A dependent claim adds limitations to its parent, so falling outside the independent claim means falling outside every claim that depends on it.
A patent with twenty claims may have only two independent ones, which substantially reduces the analysis.
The doctrine of equivalents
A missing element does not always end the matter.
| Test | Question |
|---|---|
| Function-way-result | Same function, in substantially the same way, for the same result? |
| Insubstantial differences | Is the difference insubstantial to a skilled person? |
It applies element by element, not to the invention as a whole. You cannot argue the products are equivalent overall; you must show the specific substituted element is equivalent to the specific claimed one.
Prosecution history estoppel limits it heavily. Claim scope surrendered during examination to overcome prior art generally cannot be recaptured through equivalents. See doctrine of equivalents.
Which is why reading the file history matters before concluding that a design-around works.
Where and when it happens
Territory and timing are threshold questions, and both are easy to check.
| Question | If no |
|---|---|
| Is the act in the US? | No US infringement |
| Is the patent in force there? | No infringement |
| Was the act during the term? | No infringement |
| Is there a US patent at all? | Foreign patents give no US rights |
Rights are national. A product manufactured and sold entirely in one country cannot infringe another country's patent, however identical it is.
The term question is more nuanced than expiry. A patent that lapsed for unpaid fees was not in force during the lapse, so acts during that window did not infringe — even if the patent was later revived, though intervening rights then apply.
Direct versus indirect
| Type | Statute | Requires knowledge? |
|---|---|---|
| Direct | §271(a) | No |
| Induced | §271(b) | Yes — knowledge and intent |
| Contributory | §271(c) | Yes |
| Component export | §271(f) | Yes |
| Product of a patented process | §271(g) | — |
Indirect infringement requires an underlying direct infringement by someone. Inducement without anyone actually infringing is not actionable.
Knowledge is the dividing line. Direct liability attaches regardless of what you knew; indirect liability requires that you knew of the patent and intended the infringing use. See contributory patent infringement.
Method claims and divided infringement
Performing every step infringes a method claim. The difficulty arises when different parties perform different steps.
| Situation | Liability |
|---|---|
| One party performs all steps | Direct infringement |
| Two parties, no relationship | Generally no liability |
| One party directs or controls the other | Attributed to the directing party |
| Joint enterprise | Attributed |
Divided infringement is a real defence for systems where a server performs some steps and a user performs others. Establishing direction or control is a high bar.
Which is a drafting lesson. Method claims written so that a single actor performs every step are far more enforceable than those splitting steps across parties.
Reading a claim properly
Claims have a structure, and reading them in order makes the comparison mechanical.
| Part | Example | Function |
|---|---|---|
| Preamble | "A valve assembly comprising:" | Context; sometimes limiting |
| Transition | "comprising" / "consisting of" | Decides whether extras matter |
| Body | The elements, usually listed | What must be present |
The transition word is the one people miss. "Comprising" is open — extra elements do not avoid infringement. "Consisting of" is closed — extra elements can avoid it.
Almost all claims use "comprising", which is why adding features never helps.
Break the body into a numbered list and check each item against the product separately. That is the whole of an infringement comparison.
Exhaustion
An authorised first sale exhausts the patent holder's rights in that item.
| Scenario | Exhausted? |
|---|---|
| Bought from the patent holder | Yes |
| Bought from an authorised licensee | Yes |
| Bought abroad, imported | Yes — Impression Products v. Lexmark |
| Bought from an unauthorised maker | No |
| Repaired | Generally permitted |
| Reconstructed | Not permitted — that is making |
Repair versus reconstruction is the recurring dispute. Replacing a worn part is permissible repair; rebuilding the article essentially anew is making a new infringing product.
International exhaustion applies since Lexmark. A patent holder's authorised sale abroad exhausts US rights in that item.
The transition word
| Transition | Meaning | Extra elements |
|---|---|---|
| "Comprising" | Open | Do not avoid infringement |
| "Consisting of" | Closed | Can avoid it |
| "Consisting essentially of" | Partly closed | Depends on materiality |
Almost every claim uses "comprising", which is why adding features never helps. Checking the transition word takes seconds and settles the most common misunderstanding in the field.
Defences
| Defence | Basis |
|---|---|
| Non-infringement | An element is missing |
| Invalidity | The claim should not have issued |
| Unenforceability | Inequitable conduct during prosecution |
| Exhaustion | Authorised first sale |
| Licence | Express or implied |
| Prior user rights | §273, commercial use over a year before filing |
| Laches and estoppel | Delay and reliance, in limited circumstances |
| Expiry | The patent is no longer in force |
Expiry is the cheapest defence and the first to check. Nearly three in five US utility patents lapse before term — see the patent survival curve.
Invalidity requires clear and convincing evidence in court, a materially higher standard than the preponderance applied at the PTAB, which is why documentary prior art often goes to inter partes review instead.
Research and regulatory exceptions
| Exception | Scope |
|---|---|
| Common law experimental use | Very narrow — idle curiosity or amusement |
| §271(e)(1) safe harbour | Broad — activity reasonably related to FDA approval |
The common law exception protects almost nothing. Research with any commercial purpose falls outside it, including university research tied to funding or licensing.
The regulatory safe harbour is genuinely broad, and it is what allows generic manufacturers to develop and test before patent expiry. See pharmaceutical patents.
Building the comparison
A claim chart is the standard work product, and it is the same tool whether you are asserting or defending.
| Column | Contains |
|---|---|
| Claim element | The exact words, one element per row |
| Accused feature | The specific corresponding feature |
| Evidence | Manual page, teardown photo, source excerpt |
| Present? | Yes / no / arguable |
Credibility rests on the evidence, not the argument. Public documentation, manuals and specifications carry weight; assertions do not.
Build it before forming a view. The discipline of one row per element prevents the overall-similarity reasoning that produces wrong answers. See patent claim chart.
Worked example: three products, one claim
Claim 1 recites: a housing; a spring-biased poppet within the housing; and a bypass channel positioned to equalise pressure across the poppet during transition.
| Product A | Product B | Product C | |
|---|---|---|---|
| Housing | Yes | Yes | Yes |
| Spring-biased poppet | Yes | Magnetically biased poppet | Yes |
| Bypass channel for transition equalisation | Yes | Yes | No bypass at all |
| Extra features not claimed | Yes, several | — | — |
| Literal infringement | Yes | No | No |
| Doctrine of equivalents | — | Arguable | No — element absent entirely |
Product A infringes despite having more features. Extras never avoid infringement.
Product B is the equivalents question. Is magnetic biasing insubstantially different from spring biasing? If the applicant narrowed to "spring-biased" during prosecution to overcome prior art, estoppel likely bars the argument.
Product C does not infringe. The bypass channel is simply absent, and no doctrine bridges a wholly missing element.
When infringement is not worth pursuing
| Factor | Weighs against asserting |
|---|---|
| Short remaining term | Case may outlast the patent |
| Weak claims after prosecution narrowing | Construction likely unfavourable |
| Strong prior art in the field | Invalidity risk |
| Defendant can design around cheaply | They will, and pay nothing |
| No marking compliance | Pre-notice damages lost |
| Small damages base | Cost exceeds recovery |
Design-around cost caps what a patent can extract. If a competitor can avoid the claim for a modest engineering change, no assertion will be worth more than that.
Check the term before anything else. Litigation to judgment takes two to four years. See patent litigation.
Damages exposure if you do infringe
| Measure | Basis |
|---|---|
| Reasonable royalty | Statutory floor, §284 |
| Lost profits | Where the holder would have made the sales |
| Enhanced damages | Up to treble, for willfulness, discretionary |
| Pre-notice period | Limited by marking or actual notice |
| Injunction | Four-factor equitable test |
Marking limits the clock. A patent holder who practises the invention and does not mark generally cannot recover for the period before actual notice. See patent marking.
Willfulness is about conduct after notice. Continuing without a reasoned basis supports enhancement; a written opinion obtained beforehand is the principal defence.
If you are accused
| Step | Why |
|---|---|
| 1. Confirm the patent is in force | It may have lapsed |
| 2. Read the independent claims yourself | Letters overstate scope routinely |
| 3. Compare element by element | The all-elements rule is your first defence |
| 4. Read the prosecution history | Narrowing limits construction and estops equivalents |
| 5. Search for prior art | Invalidity, and IPR eligibility |
| 6. Diarise the one-year IPR bar | From service of a complaint |
| 7. Assess design-around cost | Often the cheapest resolution |
| 8. Get a written opinion | Defence against enhanced damages |
Do not respond before doing steps one to four. They take a day and frequently change the position entirely.
Timing and the damages window
| Event | Effect on the window |
|---|---|
| Patent issues | Infringement becomes possible |
| Publication of the application | Pre-grant royalties may accrue |
| Marking by the holder | Constructive notice |
| Actual notice given | Damages start, if unmarked |
| Six-year limit | Recovery capped at 6 years back, §286 |
| Patent expires | Infringement ends |
Damages reach back six years at most under §286, regardless of how long infringement continued.
Marking is what makes the earlier period recoverable. Without it, the window usually starts when the letter arrived. See patent marking.
Infringing a patent: the checklist
- Confirm the patent is in force before any analysis. Lapse is the cheapest answer.
- Work from the claims, never the title, abstract or description.
- Start with the independent claims. Avoiding them avoids everything dependent.
- Apply the all-elements rule. One missing element defeats literal infringement.
- Remember extra features never help. Containing everything claimed plus more still infringes.
- Check the prosecution history for narrowing that estops equivalents.
- Consider the acts, not just the product — offering to sell and importing are separate infringing acts.
- Check exhaustion if the item was bought from an authorised source.
- Assess whether the invention is used in the US at all, since rights are national.
- Get a written opinion before continuing a known risk. Direct infringement needs no knowledge, but enhanced damages do.