A patent can be infringed in two basic ways: directly and indirectly. These different ways of infringing are important to understand by anyone wanting to enforce patents and by anyone wanting to avoid infringing a patent.

Direct Infringement

A patent claim is directly infringed by some product when that product practices each "limitation" of the claim when the claim is a method claim, or includes a structure equivalent to each "limitation" of the claim when the claim is an apparatus claim, or does both in the case of a means-plus-function claim. The term "limitation" is used by lawyers and those in the know to mean the step or steps spelled out in the patent claim.

There is another way of directly infringing known as the doctrine of equivalents. If a product doesn't perfectly meet the limitations of a claim, it may still infringe on the patent if it does something that a person of ordinary skill in the art ("POSITA" or "OOSITA") would know was equivalent. While this may seem like a loophole, it's actually quite fair. Suppose someone patents a "glass bulb containing a filament that produces light when an electric current is sent through it." The inventor may not realize that a bulb made of specially treated plastic would also work. The key to the invention isn't the material from which the bulb is made. Thus a plastic bulb could infringe on the patent because of the doctrine of equivalents.

Indirect Infringement

If a product doesn't directly infringe a patent, it may also indirectly infringe. There are two types of indirect infringement: induced infringement and contributory infringement. Encouraging others to infringe a patent is called "induced infringement." When two products working together infringe on a patent, that's called "contributory infringement."

Induced infringement

Induced infringement occurs when some party purposefully causes or encourages another party to infringe a patent. The key work here is "purposely." Induced infringement must meet these criteria:

• The infringer actively encouraged or instructed another party on how to use a product or perform a process in a way that infringes the patent claims.
• The infringer knew of the patent.
• The infringer knew or should have known that the encouragement or instructions would induce infringement of the patent.
• The other party actually infringed the patent.

So induced infringement requires that the infringer purposely got another party to infringe a patent. Let's say you have a software company that produces word processing software, and your competitor has a patent on checking the spelling of words in a document and putting a squiggly red line under misspelled words. You need a spell checker, but you don't want to infringe, so your product outputs misspelled words to a separate file and you recommend that your customers buy third-party software that reads in that separate file and puts squiggly red lines in the documents. You may be inducing infringement by encouraging your customers to perform the patented method. And don't try to use an orange line or a chartreuse line, because you'll probably still be infringing under the doctrine of equivalents.

Contributory Infringement

Contributory infringement occurs when one party who knows about the patent supplies a component to another party to use in an infringing product. Contributory infringement occurs if the party who received the component directly infringes the patent and if the component has the following characteristics:

• The component is a significant part of the invention.
• The component is especially made for use in a way that infringes the patent.
• The supplier knows that the component was especially made for that use.
• The component doesn't have any substantial non-infringing use.

So contributory infringement requires that the infringer knowingly supplied another party with a product that was incorporated into a second product, causing the combination to infringe a patent.

Let's take the example again of your word processing company. Suppose your word processing company decides to buy the third-party squiggle generator software to incorporate into your word processor. You meet with the head of the squiggler company and tell her that your word processor doesn't infringe on your competitor's product and the squiggler doesn't infringe. You want to embed the squiggler into the word processor. At that point you've just lost the game, because the squiggler company knows that the combination infringes and so they can be found to be contributory infringers. Because it's your product, you can be liable for direct infringement in this scenario.

However, if the squiggler can be used to underline important words, hyperlinks, and naughty words in addition to misspelled words, then the squiggler company may be OK because the component has significant uses that are non-infringing.