Kha is the second of the thirty consonants and the aspirated partner of ka: the same closure at the soft palate, released with a breath the ear can hear. It takes the ya, ra and wa subscripts, and like ka it plays no part in the prefix, suffix or superscript systems.
The pairing is not local to the velar line. Tibetan inherited from the Indian ordering of sounds a grid in which each line of stops runs unaspirated, aspirated, voiced, nasal, and that shape repeats through the palatals, the dentals, the labials and the affricates. Kha stands in the second column, and so do cha, tha, pha and tsha directly beneath it. A learner who grasps the column has grasped five letters rather than one, which is why the alphabet is taught as a recitation with a rhythm rather than as a list to be memorised item by item.
Aspiration was a genuine contrast in the classical language and it remains one in the modern language, but it no longer works alone. In Lhasa speech the old unaspirated ka and the aspirated kha have fallen together as an aspirated k, and both carry high tone. The distinction the second column preserves is now a fact of spelling and of the older language rather than a difference a Lhasa speaker produces. Other dialects, particularly in Amdo, keep more of the older system, so what has merged depends on where the speaker is from.