This is  the  only  Indo-European language to  exhibit the  triple reflex, the  realization of  the  three laryngeals interconsonantally as three different vowels.
This language is the only one in its family to split original labiovelars into velars when adjacent to high back vowels, dentals before some front vowels, and labials otherwise. The early history of this language saw a chain shift of the low-mid, high-mid, and high back unrounded vowels towards a new high front rounded vowel. This is  the  only  Indo-European language to  exhibit the  triple reflex, the  realization of  the  three laryngeals interconsonantally as three different vowels. One phoneme in the (*) classical variety of this language had a realization that is disputed between [dz] and [zd], while the more recent history of this language's vowel system has seen a series of mergers into the high front vowel. This language was divided until the late twentieth century into demotiki ["thee"-MOW-tee-tsee] and Katharevousa varieties, the first of which evolved from Koine. For 10 points, name this European language, first attested in the Linear B script, that once included Attic and Ionic varieties. 
This language is the only one in its family to split original labiovelars into velars when adjacent to high back vowels, dentals before some front vowels, and labials otherwise.
Greek