The Three-Letter Rule  

The “three-letter rule” is a spelling convention stating that “content words”—words that communicate meaningful information, such as nouns, (most) verbs, adjectives, and adverbs—will almost always be spelled with at least three letters. Words that are spelled with only one or two letters, on the other hand, will almost always be “function words”—words that perform grammatical functions to help construct a sentence, such as pronouns, prepositions, conjunctions, articles, or particles.

The rule is most helpful when we are trying to identify content words that are homophones with function words (that is, they are pronounced the same way but are spelled differently). First, though, let’s look a bit more closely at the difference between content words and function words.

A content word (also known as a lexical word) is a word that communicates a distinct lexical meaning within a particular context—that is, it expresses the specific content of what we’re talking about at a given time. Nouns (e.g., dog, Betty, happiness, luggage), most* verbs (e.g., run, talk, decide, entice), adjectives (e.g., sad, outrageous, good, easy), and adverbs (e.g., slowly, beautifully, never) all have meaning that is considered lexically important.

In addition, content words are considered an open class of words, meaning that new words can be added to their various parts of speech without difficulty. For example, the word email is now an accepted and commonplace term, with a distinct lexical meaning as both a noun and a verb, but it wouldn’t have made sense to anybody 100 years ago.

Finally, content words will always have at least one syllable that is emphasized in a sentence, so if a content word only has a single syllable, it will always be stressed. (Single-syllable function words like a or of, on the other hand, are very often—though not always—unstressed in speech.)

(*Auxiliary verbs are specific types of verbs that are used in the grammatical construction of tense and aspect or to express modality—that is, asserting or denying possibility, likelihood, ability, permission, obligation, or future intention. These types of verbs are fixed in their structure and are used to convey a relationship between other “main” verbs, so they are considered function words, which we’ll look at next.)

A function word (also known as a structure word) is a word that primarily serves to complete the syntax and grammatical nuance of a sentence. These include pronouns (e.g., he, she, it, they), prepositions (e.g., to, in, on, under), conjunctions (e.g., and, but, if, or), articles (e.g., a, an, the), other determiners (e.g., this, each, those), and interjections (e.g., ah, grr, hello).

In addition to these other parts of speech, function words also include a specific subset of verbs known as auxiliary verbs, which add structural and grammatical meaning to other main verbs. These include the three primary auxiliary verbs be, do, and have, as well as a number of others known as modal auxiliary verbs, such as can, may, must, will, etc. For example:

Another distinct aspect of function words is that they are considered a closed class of words, which, unlike content words, means that we don’t (or very rarely) add new words to these groups. For instance, we generally cannot create a new preposition to describe a relationship between a noun and the rest of the sentence, just as we have specific conjunctions that already exist to connect words, phrases, or clauses together. (Note that interjections are an exception to this, as they can evolve or be added to over time.)

Finally, single-syllable function words are commonly (but not always) unstressed in a sentence—since they are not providing lexical meaning integral to the sentence, we often “skip over” them vocally. For example, in the sentence, “Bobby wants to walk to the playground,” the particle to, the preposition to, and the definite article the are all said without (or without much) stress. The content words (Bobby, wants, walk, and playground), on the other hand, each have at least one syllable that is emphasized.

The three-letter rule is a useful convention to follow when we’re trying to determine the spelling of short, single-syllable words. Many one- and two-letter function words are homophones of short content words: they have different spellings, but their pronunciations are the same. (We’ll also look at a few instances in which the spelling of such words is similar in all but one letter, but their pronunciations are affected by the change in addition to their meaning.)

By remembering that content words will almost always be three or more letters long, we can choose the correct spelling for the word we mean.

For example:

Even where a short content word does not have a homophonic function word from which it needs to be distinguished, we still commonly find silent, seemingly extraneous letters in three-letter words that would have the same pronunciation with only two letters. We often find this when the final consonant of the word is doubled; other times, silent E or other silent vowels are added at the end of the word to “reinforce” the main vowel’s pronunciation. For example:

While the three-letter rule is fairly reliable for determining the spelling of common words, there are a few content words that have fewer than three letters.

The most common content words that go against the three-letter rule have two letters—the only one-letter words in English are a (an article), I (a pronoun), and O (an archaic interjection).

However, it’s important to note that several of these two-letter content words are actually abbreviations of other words, and are often considered informal or nonstandard; others are colloquial terms, or are specific to a certain field, practice, or study. The number of two-letter content words used in everyday speech and writing is actually quite small.

Here is a full list of examples:

In addition to the examples we looked at above, individual letters of the alphabet can operate as nouns when we talk them in a sentence, as in, “There is no i in team.”

However, the appropriate way of writing single letters can be a thorny issue. They can often look incorrect if they are simply written as a lowercase letter with no other punctuation or formatting, so a common solution is to surround a single letter in quotation marks, as in:

The biggest problem, though, is how to write plurals of single letters. If a single letter is lowercase, then simply adding “-s” to the end can create confusion with existing words, as in:

In these cases, surrounding the letter in quotation marks will not help. For instance, writing “is” does nothing to clear up the confusion with the word is, and writing “i”s is both incorrect and visually unappealing.

To combat this potential confusion, it is common to use an apostrophe between the alphabetical letter and the pluralizing “-s” without quotation marks, as in:

However, this use of apostrophes is sometimes frowned upon by linguists who feel the apostrophe should be reserved for possession or contraction. As a result, it is common for writers to capitalize single letters so that they are distinct from the “-s.” For example:

Some writers will still opt to use apostrophes to make their writing even clearer, even when a letter is capitalized:

In the end, there is no single, standardized way of writing single letters of the alphabet in a sentence; unless the style guide of your school or employer specifies how to write them, choose the style that you feel looks the best and is the least ambiguous. Whichever method you decide to use, just be consistent throughout your writing.

Finally, it’s also worth mentioning that letters of the alphabet can be used as adjectives in certain situations to describe the shape of something. These generally occur in very specific instances. The letter is almost always capitalized, and the terms are often (but not always) conjoined by a hyphen. For example: