Nope, there’s no aluminum foil today, and I’m not trying to foil your best laid plans. But we will discuss a very important literary device that you may want to consider for your own writing.
Let’s do a very quick tour of etymology. The word foil actually comes from the Latin for blade, which can refer to both a sword or a plant (think foliage), and may be related to the fencing sword called a foil. How does that apply to literature? Well, in fiction, a foil is someone (most often) or something (less so) that helps to cut away the ambiguity and draw out another character in sharper relief.
Technically, the term is sometimes used of the antagonist of the story (who is in sharp contrast to the hero), but a foil can simply be the plucky sidekick who helps the audience see the genius of the main character. Dr. Watson is the perfect example of this because he never understands how Sherlock Holmes solves his cases—and neither would we if Watson didn’t insist on understanding the deduction.
A foil can also be more than a character, however. For example, some plot element could help the audience see the story more clearly. A dream might reveal something about what’s happening that the character doesn’t see at first, or a story within the story might mirror the plot.
The main point to remember is that a foil is anything that helps the reader understand what’s going on by drawing a contrast (e.g., someone who knows vs. someone who doesn’t). This makes the foil an important factor in writing.
Have some examples of foil characters or devices? Leave them in the comments.