The Centum – Satem Split
Assibilation is a sound change resulting in a sibilant consonant. It is a form of spirantization and is commonly the final phase of palatalization.
A labiovelar is a velar consonant that is labialized, with a /w/-like secondary articulation. Common examples are [kʷ, ɡʷ, xʷ, ŋʷ], which are pronounced like a [k, ɡ, x, ŋ], with rounded lips, such as the labialized voiceless velar plosive [kʷ].
Plain Velars are consonants articulated with the back part of the tongue (the dorsum) against the soft palate, the back part of the roof of the mouth (known also as the velum).
Palatovelars – since the velar region of the roof of the mouth is relatively extensive and the movements of the dorsum are not very precise, velars easily undergo assimilation, shifting their articulation back or to the front depending on the quality of adjacent vowels. They often become automatically fronted, that is partly or completely palatal before a following front vowel, and retracted, that is partly or completely uvular before back vowels.