Frontal Lisp: What It Is and the Age It Should Resolve

✓ Reviewed against ASHA Practice Portal, ASHA developmental milestonesEvidence level ALast reviewed September 29, 2026Published July 8, 2026

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.

At what age should a frontal lisp go away?

The short answer: a frontal (interdental) lisp can be a normal part of development while S and Z mature, and most children produce a clear /s/ by around age 4 to 5 — in the largest U.S. review, 90% of children had acquired /s/ during the fifth year. A frontal lisp still going strong past about age 4½ to 5 is the one to have assessed, because past that point it is less likely to fully self-correct.^9,4

Frontal vs. lateral lisp at a glance.
	Frontal (interdental) lisp	Lateral lisp
What it sounds like	S sounds like “th” — “thun” for “sun”	Wet, “slushy” S with air over the tongue’s sides
What the tongue does	Pushes forward between the front teeth	Lets air escape sideways instead of down the middle
Is it developmental?	Often, up to about age 4½	No — not typical at any age
Typical outlook	Frequently fades; responds quickly to practice	Rarely resolves alone; needs an SLP to shape

Frontal vs. lateral lisp at a glance.¹

Unsure which one you are hearing? The free lisp test below records three S-words and plays each next to a clear example, so you can hear whether the S is “th”-like or slushy before deciding what to do next. It is a screening, not a diagnosis.

Free lisp test — is the S “th”-like or slushy? Two minutes, for a child or yourself
Lisp test words and sentences — S and Z in every position

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

Age 4½the rough point past which a lingering frontal lisp is worth checking.⁴

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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.

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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 Test

Say three S words, hear them beside clear examples and listen for the “th”-like S of a frontal lisp. For a child or yourself, about two minutes.

Try it free →

When to see a speech-language pathologist

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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.

What is the age of elimination for a frontal lisp?＋

There is no single official “age of elimination,” but the norms behind the question are clear: in the largest review of U.S. acquisition studies, 90% of children acquired /s/ and /z/ during the fifth year (between 4;0 and 4;11), and ASHA’s milestones for 4-to-5-year-olds describe mostly clear speech. In practice, clinicians treat about age 4½ to 5 as the point past which a lingering frontal lisp warrants an assessment rather than more waiting.

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.