Semi-, Bi-, and Twice-

These are always confusing. In short, I would always ask someone to clarify what they mean when they use one of these prefixes.

However, here are the actual definitions –

Semi is Latin for “half,” so to say something happens “semi-monthly” is to say that it happens “every half month,” or twice a month. This is confusing because we naturally associate semi with “kinda, sorta,” and to kinda, sorta do something every month, could mean to do it every other month.

The technically correct usage of “semi” is not an arguable point, but the incorrect usage is extremely common. Therefore, it’s equally likely that someone could be using it incorrectly as it is that they’re actually using it correctly, so always clarify.

Even when used “correctly,” Bi is unavoidably ambiguous. “Bi-annual” could legally mean one of two things:

1. Twice a year
2. Every two years

The problem is determining which noun that “bi-” modifies. Does it refer to the “annual” part – so every two years – or does it refer to the (implied) event part – so twice every year? What are we doubling up – the event or the time period? There’s no way to tell someone’s intention without asking.

Twice is probably the safest prefix to use. If we say something is “twice-yearly,” it’s clear (for whatever reason) that “twice” refers to the event, not the time period.

Why I Looked It Up

I just always wondered.