I created a recurring reminder in my iPhone the other day, and I set it to repeat Every 2 Weeks. The calendar app shortened it to Biweekly, and I thought, Wait, is that right? Which word means twice a month: biweekly or bimonthly?

It turns out I’m not the only person who has trouble with this distinction. According to Merriam-Webster’s Collegiate Dictionary, “Many people are puzzled about bimonthly and biweekly, which are often ambiguous because they are formed from both senses [‘every two’ and ‘two times’] of bi-. This ambiguity has been in existence for nearly a century and a half and cannot be eliminated by the dictionary.”

Okay, if the dictionary can’t solve it, what are we supposed to do? Some usage guides suggest using bi- to mean “every other” and semi- to mean “twice a.” This technique works fairly well for weeks and months, but it falls apart at years.

*What’s the difference between biweekly and semimonthly? This comes up most often with payroll. A company that issues paychecks every other Friday pays biweekly, and a company that issues paychecks on the first and fifteenth pays semimonthly.

Even if you follow this usage guide exactly, you face a problem: Your audience probably doesn’t know these distinctions. As Merriam-Webster points out, “The chief difficulty is that many users of these words assume that others know exactly what they mean, and they do not bother to make their context clear.”

Your best bet, then, is to skip these words altogether and just use the phrases in the right-hand column (“twice a month,” “every other year,” etc.). Everyone will understand what you mean—including you.