Differences between different spellings of English words come from a lot of sources: different meanings, double form acceptance, American or British English variations, diversity of prefixes or suffixes and so on.
But no matter their cause, so many similar words can become confusing and can make you ask yourself whether one or the other form is correct. Let's see in which case we find "flyer" and "flier" and how to use them correctly according to their meaning.
Flyer vs. Flier
Both "flyer" and "flier" are English nouns and, despite their different spelling, they both have the same definition. "Flyer" refers to a piece of paper with small dimensions, used to advertise an event, a business, a show etc. Secondly, the same word can define a flying person, somebody who flies (for example, someone on an aircraft).
"Flier" is just another spelling of the same word. It carries the same signification as "flyer" and it is also accepted as a correct version in the English vocabulary.
When do we use "flyer"?
You can use flyer when you refer to a small advertising paper. Or, of course, you can also use it with its other sense, when you refer to a passenger on an aircraft. The only rule is to use it as a noun.
When do we use "flier"?
As a slightly different spelling for the same word, "flier" can be used in all contexts where you use "flyer", with the same meaning. They are replaceable.
The difference between "flyer" and "flier" is not related to British/American English variations, to different meanings of the words or other similar reasons. They are just spelled differently and mean the same. Consequently, you have no restriction regarding their use. You can replace "flyer" with "flier" and vice versa without changing the meaning of your communication.