The Three-Letter Rule  

What is 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.

Content Words vs. Function Words

Content 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.)

Function words

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:
  • “I am going home.”
  • “We do not agree with the outcome.”
  • “You must tell me what we can say during the interview.”
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.

Determining spelling using the three-letter rule

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:
Function Words
Content Words
aw (/ɔ/)
interjection used to express pity, sympathy, tenderness, or disbelief (sometimes spelled aww)
awe (/ɔ/)
noun huge or overwhelming respect, wonder, or admiration, sometimes mixed with dread or fear
be* (/bi/)
auxiliary verb used to denote grammatical, tense, aspect, mood, or passive voice
bee (/bi/)
noun a winged insect capable of stinging that collects nectar and pollen
by (/baɪ/)
preposition close or next to; via or with the help of; through or due to the agency, action, or invention of
buy (/baɪ/)
verb to purchase with money
do* (/du/)
auxiliary verb used to ask questions or form negatives
dew (/du, dju/)
noun droplets of water that condense onto cool surfaces from the air, especially at night
due (/du, dju/)
adjective payable on demand or owed as a debt; conventional, proper, or fitting
doe (/doʊ/)
noun a female deer, or the female of various other non-primate mammals
er (/ə, ər/)
interjection expressing hesitation, doubt, or uncertainty
err (/ɛr/)
verb to make an error; to do what is incorrect, improper, or unacceptable
ere (/ɛr/)
preposition/conjunction (old-fashioned) before; prior to
ew (/ɪəu/)
interjection an exclamation of disgust (sometimes spelled eww)
ewe (/ju/)
noun a female sheep
hi (/haɪ/)
informal interjection used to express greeting
high (/haɪ/)
adjective and adverb elevated or extended to a relatively great degree upward
in (/ɪn/)
preposition/particle within the area of; into from the outside (among other meanings)
inn (/ɪn/)
noun a lodging house, tavern, or restaurant
I (/aɪ/)
first-person singular pronoun (subjective case)
aye (/aɪ/)
noun an affirmative vote or response
eye (/aɪ/)
noun the organ responsible for vision
lo (/loʊ/)
interjection used to express surprise or attract attention, largely reserved in modern English for the phrase lo and behold
low (/loʊ/)
adjective or adverb near to or rising only slightly above the ground; not high or tall
of (/ʌv/)
preposition originating, derived, or resulting from
off (/ɔf/)
adjective not connected, attached, on, or operational; adverb from or at a place, position, or certain distance (can be a preposition as well, used to indicate the removal, detachment, or distance of something)
or (/ɔr/; when unstressed, /ər/)
conjunction used to indicate one or more choices or alternatives
oar (/ɔr/)
noun a long, thin paddle used to row or steer a boat
ore (/ɔr/)
noun a naturally occurring mineral, typically metal, mined for its value
ow (/aʊ/)
interjection used to express pain
owe (/oʊ/)
verb to be indebted or to have a moral obligation to
to (/tu/)
preposition towards or in the direction of; particle used to form infinitives, as in to go, to run, to walk, etc.
too (/tu/)
adverb also or in addition; immensely, extremely, or excessively
two (/tu/)
noun the name of the number 2, the sum of 1 + 1; also used as a determiner, as in “two sandwiches,” “two books,” etc.
toe (/toʊ/)
noun one of the five digits of the foot
tow (/toʊ/)
verb to pull or draw from behind; noun the act or an instance of towing
us (/ʌs/)
first-person plural pronoun (objective case)
use (v. /juz/; n. /jus/)
verb to employ for a purpose or put into practice; noun the act or an instance of using
we (/wi/)
first-person plural pronoun (subjective case)
wee (/wi/)
adjective exceptionally small; tiny
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:
*Be, do, and have as main verbs
It is worth noting that the auxiliary verbs be, do and have can also function as “main” verbs in certain situations, meaning they classify as content words depending on the context. For example:
  • “Please do your homework.”
  • “I am hungry.”
  • “We don’t have time for this.”
Do and be are not the only content words that have fewer than three letters, though. We’ll look at some other examples next.


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.

Two-letter content words

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:
  • ab (an informal noun, short for abdominal muscle)
  • ad (a noun, short for advertisement)
  • ag (an informal adjective or noun, short for agriculture/agricultural, as in, “There are many ag students,” or, “We are studying ag.”)
  • am (first-person singular conjugation of be when used as a main verb, as in, “I am angry.”)
  • be (when used as a main verb, as in, “Please be careful.”)
  • bi (an informal adjective, short for bisexual)
  • bo (a long staff weapon used in Japanese martial arts; more formally written , transcribed from Japanese)
  • do (when used as a main verb, as in, “I have work to do.”)
  • ec (an informal noun, short for economics, as in, “I am studying home ec this semester.”)
  • ed (informal noun, short for education, as in, “I am studying special ed this semester.”)
  • el (the name for the letter L)
  • em (the name for the letter M; also used as an adjective meaning “the length of an M,” as in the term em dash)
  • en (the name for the letter N; also used as an adjective meaning “the length of an N,” as in the term en dash)
  • ex (a nominalization of the suffix “ex-,” used to refer to a former husband, wife, partner, etc., as in, “I ran into my ex the other day”; also the name for the letter X)
  • go (main verb, meaning “to move, travel, or depart”)
  • id (in psychoanalytic theory, the part of the psyche that represents unconscious and primitive impulses and desires)
  • is (third-person singular conjugation of be when used as a main verb, as in “She is angry.”)
  • lo (stylized shortening of low, as in lo-fi or lo calorie)
  • ma (colloquial term for mother)
  • no (a negative adjective, as in “I have no luck,” or adverb, as in “I did no better this time.”)
  • oh (an alternative name for the number zero, as in “My area code is three oh three” [303])
  • ox (a large domesticated bovine used as a draft animal)
  • pa (a colloquial term for father)
  • pi (the 16th letter of the Greek alphabet, π; used in mathematics as a transcendental number, approximately 3.14159)
  • up (when used as an adjective [rather than a preposition/particle], as in “Your time is up.”)

Letters of the alphabet written as words

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:
  • “There is no “i” in team.”
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:
  • “There are three is in intrinsic.” (Could be confused with the verb is.)
  • “The word aardvark actually begins with two as.” (Could be confused with the conjunction/adverb as.)
  • “Remember that pursue is spelled with two us, not one.” (Could be confused with the pronoun us.)
  • Savvy is one of the few words that contain two vs.” (Could be confused with vs., an abbreviation of the word versus.)
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:
  • “Be sure to mind your p’s and q’s while you’re with your grandparents.”
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:
  • “There are four Es in excellence.”
  • “Let me make sure that I’ve dotted my Is and crossed my Ts.”
  • Savvy is one of only a few words that contain two Vs.”
Some writers will still opt to use apostrophes to make their writing even clearer, even when a letter is capitalized:
  • “There are four E’s in excellence.”
  • “Let me make sure that I’ve dotted my I’s and crossed my T’s.”
  • Savvy is one of the few words that contain two V’s.”
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.

Alphabetic adjectives

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:
  • A-frame
  • D-ring
  • H-beam
  • I-beam
  • K-turn (more commonly known as a three-point turn)
  • O-ring
  • P-trap
  • S-bend
  • T intersection
  • T-shirt (also variously written as t-shirt, tee-shirt, and tee shirt)
  • U-bend
  • U-turn
  • Y intersection
  • Y-turn (more commonly known as a three-point turn)

1. According to the three-letter rule, what type of word will most commonly be spelled with at least three letters?

2. Which of the following is not a common characteristic of function words?

3. Which of the following parts of speech are not considered content words?

4. Which of the following is a function word?

5. Which of the following parts of speech are only sometimes considered function words?

6. When do content words contain only one letter?

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