Velar consonant

Velar consonants are the most common type of dorsal consonants, pronounced with the dorsum of the tongue raised towards the soft palate (or velum). Velarization utilizes the same gesture, but with another primary articulation. Velar or further back closure is used to produce the airflo in clicks, but their POA are generally defined by the front articulation (this also implies the impossibility of a velar click; not that it stop'd IPA from creating a symbol for one).

Back vowels may also be caracterized as velar, tho for those in the [ɔ] to [ɑ] region "uvular" or "pharyngeal" may be more accurate descriptions.

Plosives

The voiceless velar plosive /k/ is nigh-universal in languages of the world and commonly a high-frequency consonant. When aspiration or ejectiveness are contrasted, the same applies to these counterparts. Languages lacking /k/ have generally previously shifted it to /ʔ/ (examples include Tahitian and perhaps Xavante) or have other voiceless dorsal stops, usually at least /kʷ/ (sometimes /kp/ (Klao) or /q/ (Hupa)). A third possibility is a language not making any voicing distinction in stops and having [k] as an allophone of a consonant best described as /g/.

By contrast, the voiced velar plosive /g/ (or /ɡ/, if you are a stickler for proper typography) is considerably more unstable than its other "basic" counterparts /b/ and /d/; languages with a hole at /g/ are found randomly scattered around the world, with little areal relevance. Known pathways of elimination include → /ɣ/, also a likely intermediate on the road towards loss. Arabic has freak unmotivated unconditional palatalization to /ʤ/ (while in many dialects leaving /k/ be). (I think this is one of a number of hints for Classical Arabic /q/ having been [ɢ] --ed.) The implosiv /ɠ/ is subject to a similar limitation.

A curious exception of stop distribution is (Xalxa) Mongolian, which lacks /k/ but has /g/.

Affricates

Voiceless velar affricates occur mostly as an areal feature in Southern Africa, commonly furthermore being ejectiv and in some cases (Zulu) lateral. It is also fleetingly found in some High German dialects, demonstrating its role as the intermediate in the change /kʰ/ → /x/.

Voiced velar affricates are completely unattested, both as phonemic and as a predictable allophone of any other consonant.

Fricatives

Velar fricatives /x/ and /ɣ/ are not uncommon, tho not as common as their stop or labial/alveolar counterparts.

Lateral velar fricatives are possible, but rare.

Nasal

The velar nasal /ŋ/ is even more commonly lacking as a phoneme than /g/, tho for some reason its distribution is more tied to areal factors. Additionally, among languages that do have /ŋ/, there is also a strong tendency to disallow the word-initial position (examples include the Germanic languages; exceptions include Austronesian and most languages of Siberia) and sometimes any onset position (Mandarin).

Other sonorants

The labialized velar approximant /w/ (the semivowel counterpart to /u/) is common.

A velar lateral approximant /ʟ/ is very rare (a velarized alveolar /ɫ/ is somewhat more common), to the point of being about as common as its obstruent counterparts.

Sound changes involving velars

From

Velar consonants may commonly be palatalized into palatal consonants near front vowels (which may then decay via assibilation or similar pathways). This change is also attested as spontaneous in languages that contrast velars with uvular consonants, in indigenous languages of the Pacific coast of North America. This is also hypothetized of Proto-Indo-European in some reconstructions, considering that there are (AFAIK --ed.) no clear examples of palatal stops becoming velar stops.

Another typical POA change is retraction to uvular near back vowels (a much rarer spin on this occurs in Lahu, where velars become uvular before all vowels). Spontaneous backing is attested for the fricatives in many Afro-Asiatic languages.

Labialization can occur near rounded vowels, and resulting labialized velars are capable of turning to labials (eg. Greek). The change of *w → /v/, ubiquitous in continental European languages, also falls under this.

Other velar → non-velar sound changes are *w → /j/ (in Hebrew), *ɣ → /j/ (in many languages), *ŋ → /n/ (example needed)

To

Sound changes that create velars are rare. Regardless there are attested examples, including

• labialized labials → labialized velars (in Oceanic)
• linguolabials → labialized velars (in Athabascan; via a similar intermediate as the previous?)
• in languages with small consonant inventories, /p/ → /k/ (was it Iroquioan?), /t/ → /k/ (in Oceanic)
• /ʃ/ → /x/ (in Slavic (ruki), Spanish, possibly Finnic)
• /tʰ/ → /tx/ (Athabascan)
• /h/ → /x/ (probably somewhere)
• /h/ → /ŋ/ (rhinoglottophilia - some Bantu languages)
• ∅ → /k/ epenthetically between vowels (example needed)
• ∅ → /ŋ/ word-initially (Samoyedic)
• ∅ → /w/ epenthetically near rounded vowels
• creation of /w/ from back vowels by diphthongization