157. In internal combination, the initial vowel

a. In internal combination, the initial vowel or semivowel or nasal of an ending of inflection or derivation exercises no altering influence upon a final consonant of the root or stem to which it is added.

b. To this rule there are some exceptions: thus, some of the derivatives noted at 111 d; final d of a root before the participial suffix na (957 d); and the forms noted below, 161 b.

c. In external combination, on the other hand, an initial sonant of whatever class, even a vowel or semivowel or nasal, requires the conversion of a final surd to sonant.

d. It has been pointed out above (152) that in the rules of external combination only admitted finals, along with s and r, need to be taken account of, all others being regarded as reduced to these before combining with initials.