The fact that it’s named after a Maple Leaf badge still doesn’t fully explain it though, because why should the fact that it’s named after a badge change expected “leaves” to “leafs”.

One potential explanation for this phenomenon comes to us from morphology, the branch of linguistics dealing with the internal structure of words and how they are created.

First we have to introduce the idea of a “compound” and prove that “Maple leaf” is actually a compound even though it looks like two words. A compound is any word that contains two roots, that is, two “basic meaning-chunks” (that’s not a perfect definition, but it serves our purposes here). There are three usual tests to show whether something is two words or a compound:

In English specifically, compounds often get initial stress. For example, I can build my house with a black BOARD, but I write on a BLACKboard. (Note that this is not exceptionless, even in English, so this test should only be used to support the next two tests).

A second test is “semantic distance”, that is, once a compound has formed, the meaning of the compound may drift from the meanings of the original components. For “blackboard”, many of us have seen blackboards that were green, not black, passing the semantic distance test. This test is also not foolproof, however, since not all compounds show significant semantic distance yet.

The best test is “modification”. The “head” of a compound (the part of the compound that tells you what kind of word it is and how it behaves) is the only part that can be targeted for modification (by adverbs, adjectives, etc.). So, for “blackboard”, you can’t say *“That’s a very blackboard” to mean “That blackboard is very black”. That is, once “black” has become part of the compound, it can’t be modified.

So, putting all of this together, the phrase “maple leaf”, as in “There’s a maple leaf on that tree” a) has initial stress, so it passes the compound stress test, b) hasn’t changed meaning, so it fails the semantic distance test, but c) can’t be modified (“That’s a big maple leaf” can’t mean “That’s a leaf from a big maple tree”), so it passes the modification test.

Our interim conclusion is that, whether we write it with a space between it or not (which doesn’t matter at all as far a language is concerned), “maple leaf” is a single compound noun in English.

Ok, so where do we go from here?

Well, the next useful categorization is the distinction between “endocentric” and “exocentric” compounds. Endocentric compounds have the head inside the compound, which is just a fancy way of saying that the compound is a type of that thing. For example, a “doghouse” is a type of house, and a “maple leaf” is a type of leaf.

Exocentric compounds, as you might expect, are compounds that have their head outside the compound, which, again, is just a fancy way of saying that the compound is not a type of that thing. For example, “bigfoot” is not a foot, it’s a creature with a big foot. “Blackbeard” is not a beard, but a person possessing a black beard. In short, this type of compound refers to something else. That’s the “outside” part.

As it turns out, this “inside”/“outside” distinction is a useful theoretical construct that finally helps us explain “Maple Leaf”.

What differences do you note in the compound pairs below? (Because asterisks mess with formatting, I’m using X to mean “not possible/ungrammatical”)

bigfoot : *bigfeet : bigfoots
saber tooth : *saber teeth : saber tooth (when referring to the animal)

field mouse : field mice : *field mouses
salesman : salesmen : *salesmans

We get the expected irregular morphology when we have endocentric compounds, but unexpected regular morphology when we have exocentric compounds.

This is where the “outside” comes in handy theoretically. Many morphologists think that the “external” head on exocentric compounds takes up extra space in the structure of the word and consequently blocks (interferes with) irregular morphology, resulting in default, regular plural morphology instead (seen in the common English -s plural endings above).

So, [[field mouse] + PLURAL] gives “field mice”, but [[[big + foot] + (of animal)] + PLURAL] gives “Bigfoots”.

Finally we can explain “maple leaves” vs. “Maple Leafs”. Because a maple leaf is a type of leaf, the construction looks like this: [[maple leaf] + Plural], which simply gives “maple leaves”, with the irregular “v”.

For “Maple Leafs”, on the other hand, we have [[[maple leaf] + (of person, or actually badge, as I learned today)] + PLURAL], which gives “Maple Leafs”.