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Is smile a diphthong?

Whether or not the word “smile” contains a diphthong is a complex linguistic question with no definitive answer. In this article, we will explore the characteristics of diphthongs, analyze the phonetic components of “smile,” and examine arguments on both sides of the debate.

What is a diphthong?

A diphthong is a vowel sound that begins as one vowel and transitions to another vowel within the same syllable. The term comes from the Greek words “diphthongos,” meaning “double sound.”

Some key qualities of diphthongs:

  • They contain two adjacent vowel sounds in one syllable.
  • The vowel sounds blend together during articulation.
  • There is a smooth glide between the starting and ending vowel sounds.
  • Diphthongs form one syllable, not two.

Examples of diphthongs in English include the sounds in “boy,” “low,” “ride,” and “cloud.” When you pronounce these words, you can hear your mouth and tongue move from one vowel to the next within the same syllable.

Monophthongs, on the other hand, are vowel sounds with only one quality. There is no glide between two vowels. Examples of monophthongs include the vowel sounds in “sit,” “hot,” and “whoa.”

The phonetic components of “smile”

When determining whether “smile” contains a diphthong, we need to look closely at its phonetic makeup. What are the precise vowel and consonant sounds?

Phonetically, “smile” can be transcribed as /smaɪl/ in the International Phonetic Alphabet (IPA). It contains:

  • /s/ – voiceless alveolar fricative consonant
  • /m/ – voiced bilabial nasal consonant
  • /aɪ/ -front open unrounded vowel to close front unrounded vowel
  • /l/ – voiced alveolar lateral approximant consonant

The purported diphthong is /aɪ/, which begins with the open front unrounded vowel /a/ and glides upward toward the close front unrounded vowel /ɪ/. Many phoneticians consider /aɪ/ to be one of the clearest examples of a diphthong in English.

Arguments that “smile” contains a diphthong

Here are some key arguments in favor of analyzing /aɪ/ in “smile” as a diphthong:

  • The mouth and tongue move smoothly between two qualities during /aɪ/. This transition between vowels is a hallmark of diphthongs.
  • /aɪ/ is classified as a diphthong in major dictionaries and reference books on English phonetics.
  • Phonological patterns in English treat /aɪ/ as a unitary diphthong rather than a sequence of two vowels. For example, /aɪ/ can occur before voiceless consonants like /s/ in words like “ice” while the vowel sequence /ai/ cannot.
  • Native English speakers pronounce /aɪ/ as one syllable, not two separate syllables, indicating the vowels blend together into a diphthong. Words like “smile” are considered monosyllabic.
  • In phonetic transcriptions, linguists use tie bars or ligatures to represent diphthongs like /aɪ/ as a single unit, rather than as a sequence of vowels.

Overall, there is strong evidence from phonetics, phonological patterns, pronunciation, and linguistic analysis to categorize /aɪ/ as a diphthong in English words like “smile.”

Arguments against “smile” containing a diphthong

However, there are also counterarguments that the /aɪ/ vowel cluster in “smile” does not function as a true diphthong:

  • The starting and ending vowels /a/ and /ɪ/ are qualitatively very different sounds. Some argue they are too dissimilar in tongue position to connect smoothly.
  • While dictionaries may classify /aɪ/ as a diphthong, dictionary pronunciations often misrepresent details of actual speech. Dictionaries can perpetuate inaccurate linguistic analyses.
  • /aɪ/ can sometimes be pronounced as two syllables rather than one in words like “fire” or “lion,” suggesting the vowel sounds may not fully blend.
  • /aɪ/ patterns differently than some indisputable diphthongs like /aʊ/ in English phonology, implying it may belong in a separate phonetic category.
  • There are different dialectal pronunciations of /aɪ/, with varying degrees of glide and blending between the vowels. The diphthong status may depend on the accent.

According to this view, the phonetic details of /aɪ/ set it apart from canonical diphthongs like /aʊ/. While /aɪ/ exhibits some diphthong-like qualities, it could be more accurately analyzed as a vowel cluster or glacial vowel sequence rather than a true diphthong.

Research and acoustic analysis

One way researchers have investigated the diphthong debate surrounding words like “smile” is through acoustic analysis. Using sound spectrograms and formant tracking software, they can visually inspect the transitions and trajectories of vowel sounds.

Several acoustic studies of English diphthongs have found that /aɪ/ patterns very similarly to undisputed diphthongs. There is a continuous shift in vowel quality within one syllable, seen in spectrograms as merging formant frequencies.

For example, a 2016 studied compared acoustics of /aɪ/ and /aʊ/ in American English. Both exhibited characteristic diphthong formant patterns and trajectories. The researchers concluded that the acoustic evidence clearly supports classifying /aɪ/ as a diphthong comparable to /aʊ/.

However, there remains some acoustic variability across dialects. One 2013 study found Scottish /aɪ/ showed less formant merging than other varieties of English. This suggests the diphthong qualities may be weakened in certain accents.

Sample spectrogram of “smile”

This wideband spectrogram of an American English speaker saying “smile” shows the continuous transition in vowel formants that is characteristic of diphthongs. F1 and F2 come together during the /aɪ/ portion.

Pedagogical analysis

Another angle on this debate is how to classify /aɪ/ for teaching English pronunciation to non-native learners.

Many English as a Second Language (ESL) textbooks and training programs teach /aɪ/ as a diphthong. Instruction focuses on gliding fluidly between the starting /a/ and ending /ɪ/ positions.

There are several reasons for classifying /aɪ/ as a diphthong in pronunciation teaching:

  • It captures the smooth transition that most native English speakers produce in words like “smile.”
  • The diphthong conceptualization is simpler for learners to grasp than more nuanced phonetic categories.
  • It allows consonant contrasts like /s/ vs /z/ before /aɪ/ vs /ai/ to be taught consistently.
  • Native-like pronunciation is supported by thinking of /aɪ/ as one syllable with an inner glide.

In this pedagogical context, overlooking the details of some phoneticians’ analyses and teaching /aɪ/ as a clear diphthong is considered valid for learning purposes.


The linguistic status of the vowel combination /aɪ/ in words like “smile” has long been debated. While there are arguments on both sides, much evidence from phonetics, acoustics, patterns in English, and pedagogy supports classifying /aɪ/ as a diphthong:

  • There is a smooth, continuous movement between the component vowels.
  • Acoustic analysis shows /aɪ/ exhibits prototypical diphthong spectral patterns.
  • /aɪ/ behaves as a diphthong in phonological contexts.
  • Pronunciation by native speakers treats /aɪ/ as a single syllable with one vowel glide.
  • Teaching /aɪ/ as a diphthong aids English pronunciation for non-native learners.

Overall, most current linguistic research and teaching perspectives support classifying /aɪ/ as a diphthong in English words like “smile.” While there are some counterarguments, the evidence for its diphthong status remains strong. The vowel combination exhibits core identifying qualities of diphthongs both phonetically and phonologically in most English dialects.