Frontal Lisp (Interdental Lisp): Is It Normal, and How to Fix It
A frontal (interdental) lisp is when the tongue pushes forward between the front teeth on S and Z, so “sun” sounds like “thun.” It is the most common lisp and can be a normal part of development up to about age 4½. Past then, or if it is not fading, it responds quickly to teaching the tongue to stay tucked behind the teeth.
What a frontal lisp is
A frontal lisp — also called an interdental lisp — happens when the tongue pushes forward between the front teeth during the S and Z sounds. Instead of a crisp S with the air channeled down the middle of the tongue, the air spreads out and the sound turns into a soft “th.” So “sun” becomes “thun,” and “zip” takes on a “th” buzz.1
It is the most common lisp, and usually the easiest to see: you can literally watch the tongue peek between the teeth. That visibility is good news, because it makes the error simple to model and correct in a mirror.2
Is a frontal lisp normal?
Often, yes — for a while. Because S and Z keep maturing through the preschool years, a mild frontal lisp can be a normal developmental stage up to about age 4 to 4½. Most children produce a clear /s/ by around age 4 to 5.9,4
The line to watch
A frontal lisp in a 3- or 4-year-old is usually nothing to worry about. One that is still there past about age 4½ to 5 is the one worth having assessed.
How a frontal lisp is corrected
The whole fix comes down to one change: keep the tongue behind the front teeth so the air runs straight down the middle. These cues work at home and mirror what an SLP does.1
- 1Smile to close the teeth so the top and bottom teeth come almost together.
- 2Cue “tongue behind your teeth” so the tip stops poking out.
- 3Make a long, thin “snake” hiss — a finger in front of the mouth should feel a cool, narrow stream of air.
- 4Try the “long T” trick — stretch a “t” into “t-t-t-ssss” to shape the airflow from a sound your child already has.
- 5Practice in real words with a mirror — start easy, then build to phrases and everyday talk.
Skip the tongue push-ups
Non-speech mouth exercises don’t teach S. Practicing the correct S in real words is the fastest path.
Free Lisp Assessment
Check whether your child’s S is a frontal lisp — and whether to act.
When to see a speech-language pathologist
When to get an evaluation
See an SLP if a frontal lisp persists past about age 4½ to 5, if your child is hard to understand, or if home practice isn’t helping. It’s also worth a check if you are unsure whether it’s a frontal lisp or a wetter, “slushy” lateral lisp — the two are treated differently.
A certified speech-language pathologist can confirm the lisp type and target it directly, in person or by teletherapy. A frontal lisp generally responds well, and earlier is easier than reshaping a years-old habit.7,6,5
You often do not need a referral — your local school district can evaluate a child age 3 and older, and you can find a certified SLP through ASHA’s directory.8,2
Frequently asked questions
What is a frontal lisp?＋
A frontal (interdental) lisp is when the tongue slips forward between the top and bottom front teeth during S and Z, so the air spreads out and S comes out like “th” — “thun” for “sun,” “zoo” sounding like “thoo.” It is the most common type of lisp.
Is a frontal lisp normal?＋
A mild frontal lisp can be a normal part of speech development up to about age 4 to 4½, while S and Z are still maturing. Because the tongue-between-teeth position for “th” is easy and visible, many young children pass through it. A frontal lisp that lingers past about age 4½ to 5 is worth having assessed.
How do you fix a frontal lisp?＋
The fix is teaching the tongue to stay behind the front teeth so the air runs straight down the middle. A smile that brings the teeth close together, a “keep your tongue behind your teeth” cue, and starting S from a long “t” all help. Practicing the correct S in real words — with a mirror — is what makes it stick.
Will my child outgrow a frontal lisp?＋
Many mild frontal lisps resolve on their own as S develops, often by age 4½ to 5. If it hasn’t started to fade by then, it is less likely to fully self-correct — and it is quick to fix with focused practice, so getting it checked is worthwhile rather than simply waiting years.
When should I see a speech therapist about a frontal lisp?＋
See a speech-language pathologist if the lisp persists past about age 4½ to 5, if your child is hard to understand, or if home practice isn’t helping. An SLP can confirm it’s a frontal lisp (not a lateral one) and target it directly — a frontal lisp usually responds well and fairly quickly.
Put this into practice today
Try the free free lisp assessment, or start daily AI speech practice — every child takes one SpeechStep at a time.
References
9 sources from authoritative bodies. Last reviewed July 2026.
- 1.ASHASpeech Sound Disorders: Articulation and Phonology — Practice Portal page.
- 2.ASHASpeech Sound Disorders — Consumer page.
- 3.ASHATypical Speech and Language Development — Consumer page.
- 4.ASHACommunication Milestones: 4 to 5 Years — Developmental milestones.
- 5.NIDCDSpeech and Language Developmental Milestones — Fact sheet.
- 6.ASHATelepractice — Practice Portal page.
- 7.ASHAScope of Practice in Speech-Language Pathology — Policy document.
- 8.ASHAASHA ProFind — Find a Certified SLP — Clinician directory.
- 9.Peer-reviewedCrowe & McLeod — Children’s English Consonant Acquisition in the United States: A Review — Systematic review (AJSLP), 2020.