Try this for a moment: say the sound “tah”. Feel where the tip of your tongue touches the roof of your mouth, just behind your teeth. Now, say “kah”. Feel how the back of your tongue rises to meet the soft palate. Now, try to make both of those sounds at the exact same time, with a single, unified closure. Go on, give it a shot.

If you’re struggling, you’re not alone. What you just attempted is a biomechanical impossibility. Your mouth, for all its incredible versatility, is an instrument with physical limitations. It has “blind spots”—sounds and sound combinations that it is simply not built to produce. Welcome to the fascinating world of articulatory phonetics, where we map the very edges of human speech and discover why some sounds will never be part of any language on Earth.

The Articulator’s Toolkit: A Quick Tour

To understand what we can’t do, we first need a quick tour of what we can do. Producing speech is a physical act, like playing a wind instrument. Air flows from your lungs (the power source) up through your larynx (the vibrator) and into your vocal tract (the resonator). The shape of that resonator is modified by a set of highly mobile parts we call articulators.

Think of them as your built-in sound-shaping tools:

• The Lips: Crucial for sounds like /p/, /b/, and /m/ (bilabial sounds) and /f/ and /v/ (labiodental sounds).
• The Tongue: The undisputed star of the show. It’s a muscular hydrostat (like an octopus tentacle) that can change its shape in countless ways. We talk about its tip (apex), blade (lamina), body (dorsum), and root.
• The Roof of the Mouth: This isn’t one single thing, but a series of landmarks for the tongue to touch. From front to back, you have your teeth, the alveolar ridge (the bump behind the teeth), the hard palate, and the soft palate (or velum).
• The Velum: This is a muscular flap at the back of the roof of your mouth. When it’s raised, it closes off the nasal cavity, forcing air out of the mouth for oral sounds (like /b/). When it’s lowered, it allows air to pass through the nose for nasal sounds (like /m/).
• The Glottis: These are your vocal folds in the larynx. They can vibrate to produce voiced sounds (like /z/) or remain open for voiceless sounds (like /s/).

Every sound in every language is a unique combination of these parts working together. But what happens when we try to create a combination that violates the laws of physics or biology?

Mapping the Impossible: The Vocal Tract’s True Blind Spots

The unmakeable sounds fall into a few key categories, each stemming from a fundamental conflict in articulation.

Conflict of Location: You Can’t Be in Two Places at Once

This is the most intuitive limitation. A single articulator cannot be in two different places simultaneously. Your tongue tip, for example, can touch the alveolar ridge to make a /t/ or curl back to articulate a retroflex sound like the /ʈ/ found in Hindi. What it can’t do is be in both of those positions at the same time.

This is why the little experiment from the beginning failed. To make a /t/, the front part of your tongue (the apex or blade) must form a seal at the alveolar ridge. To make a /k/, the back part of your tongue (the dorsum) must form a seal at the velum. While different parts of the tongue are used, the body of the tongue can’t be bunched up in the back for a /k/ and simultaneously stretched forward for a perfect /t/. The muscular contractions required are contradictory.

Conflict of Manner: Contradictory Actions

Manner of articulation describes how the air is controlled. A stop (like /p/ or /t/) involves a complete blockage of the airstream. A fricative (like /f/ or /s/) involves forcing air through a narrow channel to create turbulence. Can you do both at once?

Not really. You can’t have a complete closure and a narrow channel at the same place, at the same time. The result of trying is an affricate, like the /tʃ/ in “church”. But notice that an affricate is a sequence: a stop immediately followed by a fricative. It’s a clever workaround, not a simultaneous contradiction.

A clearer example involves the velum. To make an oral sound, the velum must be raised, sealing off the nasal passage. To make a nasal sound, the velum must be lowered. It cannot be fully raised and fully lowered at the same time. While you can get a partially lowered velum for nasalized vowels (common in French or Portuguese), a sound that is simultaneously and fully nasal and oral is a physical impossibility.

Conflict of Airstream: The Laws of Physics

This is where things get really interesting. Some sounds are impossible not just because of anatomy, but because they defy aerodynamics.

Consider the voiced ejective. Ejectives (found in languages like Georgian and Hausa) are fascinating sounds. To make one, you close your mouth somewhere (e.g., with your lips for /p'/) and also close your glottis. Then, you raise your larynx like a piston, compressing the trapped air in your pharynx. When you release the mouth closure, the pressurized air bursts out with a sharp pop. The key is that the glottis is sealed shut.

Now consider voicing. Voicing (like the difference between /s/ and /z/) requires air to flow from the lungs through the glottis, making the vocal folds vibrate. See the problem? You cannot have a sealed glottis (for the ejective pressure) and a vibrating glottis (for the voicing) at the same instant. A voiced ejective is aerodynamically impossible.

Similarly, an implosive fricative is a non-starter. Implosives (common in languages of Southeast Asia and Africa) are made by lowering the larynx to create negative pressure in the mouth, so when the closure is released, air rushes in. Fricatives, as we know, require a sustained stream of air flowing out to create friction. A sound cannot be defined by both an inflow and an outflow of air at the same point of articulation.

The Genius of Language: Working Around the Limits

These limitations aren’t failures; they are the fundamental rules that shape the sound systems of all 7,000+ human languages. Languages don’t mourn the sounds they can’t make. Instead, they cleverly exploit the full range of what’s possible.

The Sequential Solution

As we saw with affricates, if you can’t combine two features simultaneously, just put them in order. This is the principle behind consonant clusters. No language has a “labial-alveolar” stop where the lips and the tongue tip make a single, combined sound. But countless languages have clusters like /pl/, /tr/, or /sp/, which get the job done sequentially.

The Art of Co-articulation

Sometimes, you can do two things at once, as long as you use different articulators. The English sound /w/ is a perfect example. It’s a labial-velar approximant, meaning your lips are rounded (labial) while the back of your tongue approaches the velum (velar). You’re using two distinct articulators at two distinct locations.

Some West African languages take this even further with labial-velar stops like /k͡p/ and /ɡ͡b/ (found in the name of the former Liberian president, Gbagbo). These are single sounds with two simultaneous points of closure: one at the lips and one at the velum. This is possible because the front of the mouth (lips) and the back of the mouth (tongue dorsum) can act independently.

The Path of Least Resistance

Ultimately, languages are pragmatic. The set of sounds used by any language is a tiny fraction of the total “makeable” sounds. Languages tend to select sounds that are not only easy to produce but also easy for a listener to distinguish from one another (the principle of maximal perceptual distance). The impossible sounds are filtered out by default, and even many difficult or rare-but-possible sounds (like voiceless nasals or clicks) are avoided by most languages.

The Sound of Silence

Exploring the tongue’s blind spots gives us a profound appreciation for the elegance and efficiency of human speech. Our vocal tract is not an infinite sound generator but a biological instrument, shaped by evolution and governed by physics. The sounds we can’t make are just as important as the ones we can, as they define the boundaries of the acoustic canvas on which all human languages paint their unique masterpieces.