The English language is bursting with pairs of words so similar you might think they mean the same thing, even if one has an extra syllable in the middle. Some actually do mean the same thing—disorientated, for example, is a version of disoriented more commonly used in the UK, but they both describe someone who’s lost their bearings.

Others, like systemic and systematic, have different definitions. According to Dr. Paul Brians, a former Washington State University English professor and leading authority on grammar, systematic relates to an action that is done “according to some system or organized method.” If you sort your M&Ms by color and eat the blue ones last, you’re doing it systematically. Sometimes, Brians explains on his website, systematic is used when a behavior—however unintentional it may be—is so habitual that it seems to be the result of a system. If you forget to lock your front door every time you leave the house, someone might say that you have a systematic pattern of forgetfulness.

Systemic, meanwhile, describes something that happens inside a system or affects all parts of a system. It’s often used in scientific contexts, especially those that involve diseases or pesticides. If a cancer is systemic, that means it’s present throughout the body. If you’re describing how the cancer progressed, however, you could say it spread systematically from organ to organ. As Grammarist points out, systemic can also denote something that is “deeply ingrained in the system,” which helps explain why you sometimes hear it in discussions about social or political issues. When Theodore Roosevelt served as the New York City Police Commissioner, for example, his main goal was to stamp out the systemic corruption in the police department.

In short, systematic is used to describe the way a process is done, while systemic is used to describe something inside a system.

[h/t Grammarist]