Again according to rGan Padma-rgyal-mtshan, that which one installs in the beginning is called a ka ba “pillar.” That is, perhaps in the sense of “of the [first] letter” (ka pa) as in the case of the first volume of a collection of books which is often labelled according to the first letter of the Tibetan alphabet. Cf. expression ka nas dag pa “pure from the beginning.” But according to dGe-chos, ka ba is cognate to bkar ba “to fasten.” See the dGe chos gsung ’bum (vol. 5, p. 74). That is a pillar/post is something to which other components of the house/tent is to be fastened. Note, however, that in the sMra sgo commentary ascribed by some to Rong-zom-pa, ka ba is given as example for words that have no etymology.