A voiceless alveolar fricative is a type of fricative consonant pronounced with the tip or blade of the tongue against the alveolar ridge (gum line) just behind the teeth. This refers to a class of sounds, not a single sound. There are at least six types with significant perceptual differences:

The first three types are sibilants, meaning that they are made with the teeth closed and have a piercing, perceptually prominent sound.

The voiceless alveolar sibilant is a common consonant sound in vocal languages. It is the sound in English words such as sea and pass, and is represented in the International Phonetic Alphabet with ⟨s⟩. It has a characteristic high-pitched, highly perceptible hissing sound. For this reason, it is often used to get someone's attention, using a call often written as sssst! or psssst!.

The voiceless alveolar sibilant [s] is one of the most common sounds cross-linguistically. If a language has fricatives, it will most likely have [s].^[2] However, some languages have a related sibilant sound, such as [ʃ], but no [s]. In addition, sibilants are absent from Australian Aboriginal languages, in which fricatives are rare; even the few indigenous Australian languages that have developed fricatives do not have sibilants.

The voiceless alveolar retracted sibilant (commonly termed the voiceless apico-alveolar sibilant) is a fricative that is articulated with the tongue in a hollow shape, usually with the tip of the tongue (apex) against the alveolar ridge. It is a sibilant sound and is found most notably in a number of languages in a linguistic area covering northern and central Iberia. It is most well known from its occurrence in the Spanish of this area. In the Middle Ages, it occurred in a wider area, covering Romance languages spoken throughout France, Portugal, and Spain, as well as Old High German and Middle High German.

In Romance languages, it occurs as the normal voiceless alveolar sibilant in Astur-Leonese, Castilian Spanish, Catalan, Galician, northern European Portuguese, and some Occitan dialects. It also occurs in Basque and Mirandese, where it is opposed to a different voiceless alveolar sibilant, the more common [s]; the same distinction occurs in a few dialects of northeastern Portuguese. Outside this area, it also occurs in a few dialects of Latin American Spanish (e.g. Antioqueño, in Colombia).

Amongst Germanic languages, it occurs in Dutch and closely related Low German, Icelandic, many dialects in Scandinavia, and working-class Glaswegian English.

It also occurs in Modern Greek (with a laminal articulation), as well as the Baltic languages.

There is no single IPA symbol used for this sound. The symbol ⟨s̺⟩ is often used, with a diacritic indicating an apical pronunciation. However, that is potentially problematic in that not all alveolar retracted sibilants are apical (see below), and not all apical alveolar sibilants are retracted. The ad hoc non-IPA symbols ⟨ṣ⟩ and ⟨S⟩ are often used in the linguistic literature even when IPA symbols are used for other sounds, but ⟨ṣ⟩ is a common transcription of the retroflex sibilant [ʂ].

In medieval times, it occurred in a wider area, including the Romance languages spoken in most or all of France and Iberia (Old Spanish, Galician-Portuguese, Catalan, French, etc.), as well as in the Old and Middle High German of central and southern Germany,^[3] and most likely Northern Germany as well. In all of these languages, the retracted "apico-alveolar" sibilant was opposed to a non-retracted sibilant much like modern English [s], and in many of them, both voiceless and voiced versions of both sounds occurred. A solid evidence is different spellings used for two different sibilants: in general, the retracted "apico-alveolar" variants were written ⟨s⟩ or ⟨ss⟩, while the non-retracted variants were written ⟨z⟩, ⟨c⟩ or ⟨ç⟩. In the Romance languages, the retracted sibilants derived from Latin /s/, /ss/ or /ns/, while the non-retracted sibilants derived from earlier affricates [t͡s] and [d͡z], which in turn derived from palatalized /k/ or /t/. The situation was similar in High German, where the retracted sibilants derived largely from Proto-Germanic /s/, while the non-retracted sibilants derived from instances of Proto-Germanic /t/ that were shifted by the High German sound shift. Minimal pairs were common in all languages. Examples in Middle High German, for example, were wizzen "to know" (Old English witan, cf. "to wit") vs. wissen "known" (Old English wissen), and weiz "white" (Old English wīt) vs. weis "way" (Old English wīs, cf. "-wise").

Often, to speakers of languages or dialects that do not have the sound, it is said to have a "whistling" quality, and to sound similar to palato-alveolar ʃ. For this reason, when borrowed into such languages or represented with non-Latin characters, it is often replaced with [ʃ]. This occurred, for example, in English borrowings from Old French (e.g. push from pousser, cash from caisse); in Polish borrowings from medieval German (e.g. kosztować from kosten, żur from sūr (contemporary sauer)); and in representations of Mozarabic (an extinct medieval Romance language once spoken in southern Spain) in Arabic characters. The similarity between retracted [s̺] and [ʃ] has resulted in many exchanges in Spanish between the sounds, during the medieval period when Spanish had both phonemes. Examples are jabón (formerly xabón) "soap" from Latin sapō/sapōnem, jibia "cuttlefish" (formerly xibia) from Latin sēpia, and tijeras "scissors" (earlier tixeras < medieval tiseras) from Latin cīsōrias (with initial t- due to influence from tōnsor "shaver").

One of the clearest descriptions of this sound is from Obaid:^[4] "There is a Castilian s, which is a voiceless, concave, apicoalveolar fricative: The tip of the tongue turned upward forms a narrow opening against the alveoli of the upper incisors. It resembles a faint /ʃ/ and is found throughout much of the northern half of Spain".

Many dialects of Modern Greek have a very similar-sounding sibilant that is pronounced with a laminal articulation.^[3]

This distinction has since vanished from most of the languages that once had it in medieval times.

Those languages in which the sound occurs typically did not have a phonological process from which either [s] or [ʃ] appeared, two similar sounds with which ⟨s̺⟩ was eventually confused. In general, older European languages only had a single pronunciation of s.

In Romance languages, [s] was reached from -ti-, -ci-, -ce- ([ti], [ki], [ke]) clusters that eventually became [ts], [tsi], [tse] and later [s], [si], [se] (as in Latin fortia "force", civitas "city", centum "hundred"), while [ʃ] was reached:

In High German, [s] was reached from a [t] > [ts] > [s] process, as in German Wasser vs English water. In English, the same process of Romance [ts] > [s] occurred in Norman-imported words, accounting for modern homophones sell and cell. [ʃ] was also reached from a -sk- cluster reduction as in Romance, e.g. Old English spelling "asc" for modern "ash", German schirm vs English screen, English ship vs Danish skib.

Standard Modern Greek, that has apical [s̺], lacked both processes.

The Germanic-speaking regions that did not have either phenomena have normally preserved the apical [s̺], that is, Icelandic, Dutch and many Scandinavian lects. It also reached modern times in Low German, but this language has largely been replaced by Standard German.

The main Romance language to preserve the sound, Castilian Spanish, is exceptional in that it had both events that produced [s] and [ʃ], and preserved the apical S at the expense of both, that were shifted farther away. Galician changed only [s], and Catalan, as well as Ladino, still preserves all three sounds.

Because of the widespread medieval distribution, it has been speculated that retracted [s̺] was the normal pronunciation in spoken Latin. Certain borrowings suggest that it was not far off from the sh-sound [ʃ], e.g. Aramaic Jeshua > Latin Jesus, Hebrew Shabbat > Vulgar Latin Sabato; but this could also be explained by the lack of a better sound in Latin to represent Semitic sh. It equally well could have been an areal feature inherited from the prehistoric languages of Western Europe, as evidenced by its occurrence in modern Basque.

For the same reasons, it can be speculated that retracted [s̺] was the pronunciation of Proto-Germanic s. Its presence in many branches of Indo-European and its presence particularly in the more conservative languages inside each branch (e.g. Icelandic, Spanish), as well as being found in disparate areas, such as the Baltic languages and Greece, suggests it could have ultimately been the main allophone of Proto-Indo-European s, known for ranging from [s] to as far as [ɕ].

[ʃ], but not [s], was developed in Italian. However, where Spanish and Catalan have apical [s̺], Italian uses the same laminal [s] that occurs in standard forms of English: evidence, it could be argued, that S was not pronounced apically in Latin. But Neapolitan has a medieval S becoming either [s] or [ʃ] depending on context, much as in European Portuguese, which could attest to the previous existence of [s̺] in the Italian Peninsula. The Italian pronunciation as laminal S could also be explained by the presence of [ʃ] but not [s], thus moving the pronunciation of [s̺] to the front of the mouth in an attempt to better differentiate between the two sounds.

The term "voiceless alveolar sibilant" is potentially ambiguous in that it can refer to at least two different sounds. Various languages of northern Iberia (e.g. Astur-Leonese, Catalan, Basque, Galician, Portuguese and Spanish) have a so-called "voiceless apico-alveolar sibilant" that lacks the strong hissing of the [s] described in this article but has a duller, more "grave" sound quality somewhat reminiscent of a voiceless retroflex sibilant. Basque, Mirandese and some Portuguese dialects in northeast Portugal (as well as medieval Spanish and Portuguese in general) have both types of sounds in the same language.

There is no general agreement about what actual feature distinguishes these sounds. Spanish phoneticians normally describe the difference as apical (for the northern Iberian sound) vs. laminal (for the more common sound), but Ladefoged and Maddieson^[5] claim that English /s/ can be pronounced apical, which is evidently not the same as the apical sibilant of Iberian Spanish and Basque. Also, Adams^[6] asserts that many dialects of Modern Greek have a laminal sibilant with a sound quality similar to the "apico-alveolar" sibilant of northern Iberia.

Some authors have instead suggested that the difference lies in tongue shape. Adams^[6] describes the northern Iberian sibilant as "retracted". Ladefoged and Maddieson^[5] appear to characterize the more common hissing variant as grooved, and some phoneticians (such as J. Catford) have characterized it as sulcal (which is more or less a synonym of "grooved"), but in both cases, there is some doubt about whether all and only the "hissing" sounds actually have a "grooved" or "sulcal" tongue shape.

Features of the voiceless alveolar sibilant:

The voiceless alveolar non-sibilant fricative (also known as a "slit" fricative) is a consonantal sound. As the International Phonetic Alphabet does not have separate symbols for the alveolar consonants (the same symbol is used for all coronal places of articulation that are not palatalized), this sound is usually transcribed ⟨θ̠⟩, occasionally ⟨θ͇⟩ (retracted or alveolarized [θ], respectively), ⟨ɹ̝̊⟩ (constricted voiceless [ɹ]), or ⟨t̞⟩ (lowered [t]).

Few languages also have the voiceless alveolar tapped fricative, which is simply a very brief apical alveolar non-sibilant fricative, with the tongue making the gesture for a tapped stop but not making full contact. This can be indicated in the IPA with the lowering diacritic to show full occlusion did not occur.^[82]

Tapped fricatives are occasionally reported in the literature, though these claims are not generally independently confirmed and so remain dubious.

Flapped fricatives are theoretically possible but are not attested.^[82]