For a while now, I've been thinking about a regular blog feature called "What's the Difference" wherein I'll explain the differences between two similar things. I thought I had done one years ago on this blog's ancestor the Utterly Sputter about the differences between alligators and crocodiles, but I can't find it now, so maybe I only thought about writing it.
With any luck, this will be the beginning of a new era of regular blog features. What's the Difference, installment one. Helvetica & Arial.
Arial is the Microsoft clone of Helvetica. They are extremely similar, but you can tell them apart with a few letters.
This article lays it out for your clearly. Once you've studied, or if you prefer, before you study, you can quiz yourself.