Ca is the fifth of the thirty consonants and opens the second line. It writes a voiceless unaspirated affricate formed with the front of the tongue against the hard palate, close to the ch of English cheese but without the breath that follows it in English.
Like ka before it, ca is a root letter and only that. It takes a superscript and it takes the wa subscript, but it never serves as a prefix, a suffix, a superscript or a subscript itself. In modern Lhasa speech it carries high tone, and it has fallen together with cha, the aspirated letter that follows it, in the same way that ka has fallen together with kha.
Ca matters out of proportion to its own frequency because of what is built on it. The three affricates of the fifth line, tsa, tsha and dza, are written as the shapes of ca, cha and ja carrying a small added mark. They are not inherited letters given new values; they are the palatal letters extended. So the second line is not only four letters in its own right, it is also the source of three more further down the recitation, and a learner who has the shape of ca has most of the shape of tsa as well.
The line itself runs ca cha ja nya, four letters where the Indian order it descends from has five.