The International Phonetic Association describes the Sj-sound as a "simultaneous [ʃ] and [x]", but this claim is disputed among phoneticians, including at least one former president of the IPA. Other descriptive labels include voiceless palatal-velar fricative, voiceless dorso-palatal velar fricative, voiceless postalveolar and velar fricative, or voiceless coarticulated velar and palatoalveolar fricative. The closest English phoneme is /ʃ/ ⟨sh⟩.
In Swedish phonology, the sj-sound (Swedish: sj-ljudet) is a voiceless fricative phoneme found in most dialects. It has a variety of realisations, whose precise phonetic characterisation is a matter of debate, but which usually feature distinct labialization. The sound is represented in Swedish orthography by a number of spellings, including the digraph ⟨sj⟩ from which the common Swedish name is derived as well as ⟨stj⟩, ⟨skj⟩, and ⟨sk⟩ before front vowels. The sound is transcribed ⟨ɧ⟩ in the International Phonetic Alphabet. The sound should not be confused with the Swedish tj sound [ɕ], often spelled tj or k in Swedish.
Features of the voiceless palatal-velar fricative:
- Its manner of articulation is fricative, which means it is produced by constricting air flow through a narrow channel at the place of articulation, causing turbulence.
- Its place of articulation is disputed (see below).
- Its phonation is voiceless, which means it is produced without vibrations of the vocal cords.
- It is an oral consonant, which means air is allowed to escape through the mouth only.
- It is a central consonant, which means it is produced by directing the airstream along the center of the tongue, rather than to the sides.
- The airstream mechanism is pulmonic, which means it is articulated by pushing air solely with the lungs and diaphragm, as in most sounds.
Dialects of Swedish 
This sound has been reported in certain dialects of Swedish, where it is most often known as the "sj"-sound.
Its place of articulation varies over Swedish regions and is not agreed upon (see references). It has been variously found to be the following:
- velar and postalveolar, meaning it is articulated simultaneously with the tongue dorsum (i.e. the back part of the tongue) approximating the velum (i.e. the soft palate) (like [x]) and just behind the teeth (like [ʃ]). However, doubly articulated fricatives are very difficult to pronounce or to hear, and many linguists doubt that they exist.
- Lindblad describes one of two common variants of Swedish /ɧ/ as labiodental with simultaneous velarization and protrusion of the upper lip, which would be transcribed as [fˠʷ]. (The English sigh of relief "phew!" is one approximation, as is the voiceless labial-velar approximant that is used in some varieties of English.) He does not use the symbol ⟨ɧ⟩ for this allophone.
- Lindblad describes the second common variant of Swedish /ɧ/ as velar. The difference between it and [x] is not clear, but it may have less friction ([x̞]), or be further forward ([x̟]), or both.
- A number of intermediate possibilities between these extremes.
- Other articulations have been described as well, with no obvious standard emerging.
Consider the following comments by Peter Ladefoged and Ian Maddieson:
Some dialects of Swedish have a fricative that has been said to have two or even three articulatory constrictions (Abercrombie 1967). We do not, however, think it is correct for more than one of these constrictions to be considered a fricative articulation. There is good data available on the Swedish sibilant fricatives (Lindblad 1980) allowing us to consider these sounds in detail.
The [...] Swedish fricative, usually symbolized by ɧ, is the most interesting. Lindblad describes two common variants of Swedish ɧ. The first, for which he uses a different symbol, he calls a highly rounded, labiodental, velar or velarized fricative. [...] Lindblad suggests that the source of frication is between the lower lip and the upper teeth, and it certainly appears to be so from his x-ray. He also demonstrates that the upper lip is considerably protruded in comparison with its position with that in the gesture of i. In addition to these anterior gestures, Lindblad notes that the "tongue body is raised and retracted towards the velum to form a fairly narrow constriction. (The presence of this constriction is constant, but not its width or location, which vary considerably.)" The posterior constriction in this variety of ɧ is not great enough to be itself a source of turbulence, so that, although this sound may have three notable constrictions, one in the velar region, one labiodental, and a lesser one between the two lips, only the labiodental constriction is a source of friction. The second common variant of Swedish ɧ [...] is described by Lindblad as a "dorsovelar voiceless fricative" pronounced with the jaw more open and without the lip protrusion that occurs in the other variety. Lindblad suggests that the difference between this sound and the more usual velar fricative x is that the latter "is formed with low frequency irregular vibrations in the saliva at the constriction" (Lindblad 1980, our translation). We infer from his descriptions and diagrams that this variant of ɧ has less frication, and may be slightly further forward than the velar fricative x commonly found in other languages. Lindblad claims that between the extreme positions of the labiodental ɧ and the more velar ɧ, "there are a number of intermediate types with various jaw and lip positions, including some with both anterior and posterior sound sources." [W]e doubt that it is possible to produce turbulence at two points in mouth simultaneously for ordinary linguistic purposes. The best-known case [of a possible multiply articulated fricative] is the Swedish segment that has been described as a doubly articulated voiceless palato-alveolar-velar fricative, i.e., ʃ͡x. The IPA even goes so far as to provide a separate symbol for this sound on its chart, namely ɧ. The sound in question is one variant of the pronunciation of the phonological element ʃ, which is highly variable in Swedish dialects, receiving pronunciations ranging from a palatalized bilabial sound to a velarized palato-alveolar one to a fully velar one. [I]t is not clear that any of the variants is actually a doubly articulated fricative.
— Ladefoged & Maddieson 1996:171–172, 330
The acoustic difference between [ʃ] and the Kölsch [ɧ] is difficult to perceive but the articulation is clearly distinct. Whether or not there is a relation between the Swedish /ɧ/, and the Kölsch /ɧ/, is not known. While none seems to have been established, comments (e.g. on page 18) suggest that, the choice of ⟨ɧ⟩ might well have been based upon a misunderstanding. Doubtlessly, the Kölsch /ɧ/ is not double articulated and even contrasts with a slightly velarized /ʃ/.
See also 
- Abercrombie, David. (1967). Elements of general phonetics. Chicago: Aldine.
- Garlén, Claes (1988), Svenskans fonologi : [i kontrastiv och typologisk belysning] Lund : Studentlitteratur, 1988 [Ny utg.]
- Ladefoged, Peter; & Maddieson, Ian. (1996). The sounds of the world's languages. Oxford: Blackwell Publishers.
- Lindblad, Per. (1980). Svenskans sj- och tj-ljud i ett Allmänfonetisk Perspektiv. Travaux de l'Institut de Linguistique de Lund 16. Lund: C. W. K. Gleerup.