117. Changes within the series

Of changes within the series, the most frequent and important occur in the adaptation of surd and sonant sounds to one another; but the nasals and l have also in certain cases their special assimilative influence. Thus:

a. In the two classes of non-nasal mutes and spirants, surd and sonant are wholly incompatible; no surd of either class can either precede or follow a sonant of either.

b. A mute, surd or sonant, is assimilated by being changed to its correspondent of the other kind; of the spirants, the surd s is the only one having a sonant correspondent, namely r, to which it is convertible in external combination (164 ff.).

c. The nasals are more freely combinable: a nasal may either precede or follow a mute of either kind, or the sonant spirant h; it may also follow a surd aspirant (sibilant); no nasal, however, ever precedes a sibilant in the interior of a word (it is changed instead to anusvāra); and in external combination their concurrence is usually avoided by insertion of a surd mute.

d. A semivowel has still less sonantizing influence; and a vowel least of all: both are freely preceded and followed by sounds of every other class, in the interior of a word.

e. Before a sibilant, however, is found, of the semivowels, only r and very rarely l. Moreover, in external combination, r is often changed to its surd correspondent s.

But

f. In composition and sentence-collocation, initial vowels and semivowels and nasals also require the preceding final to be sonant. And

g. Before a nasal and l, the assimilative process is sometimes carried further, by the conversion of a final mute to a nasal or l respectively.