[Footnote 159:  In practice phonetic laws have their exceptions, but more intensive study almost invariably shows that these exceptions are more apparent than real.  They are generally due to the disturbing influence of morphological groupings or to special psychological reasons which inhibit the normal progress of the phonetic drift.  It is remarkable with how few exceptions one need operate in linguistic history, aside from “analogical leveling” (morphological replacement).]

[Footnote 160:  These confusions are more theoretical than real, however.  A language has countless methods of avoiding practical ambiguities.]

More often the phonetic drift is of a more general character.  It is not so much a movement toward a particular set of sounds as toward particular types of articulation.  The vowels tend to become higher or lower, the diphthongs tend to coalesce into monophthongs, the voiceless consonants tend to become voiced, stops tend to become spirants.  As a matter of fact, practically all the phonetic laws enumerated in the two tables are but specific instances of such far-reaching phonetic drifts.  The raising of English long o to u and of long e to i, for instance, was part of a general tendency to raise the position of the long vowels, just as the change of t to ss in Old High German was part of a general tendency to make voiceless spirants of the old voiceless stopped consonants.  A single sound change, even if there is no phonetic leveling, generally threatens to upset the old phonetic pattern because it brings about a disharmony in the grouping of sounds.  To reestablish the old pattern without going back on the drift the only possible method is to have the other sounds of the series shift in analogous fashion.  If, for some reason or other, p becomes shifted to its voiced correspondent b, the old series p, t, k appears in the unsymmetrical form b, t, k.  Such a series is, in phonetic effect, not the equivalent of the old series, however it may answer to it in etymology.  The general phonetic pattern is impaired to that extent.  But if t and k are also shifted to their voiced correspondents d and g, the old series is reestablished in a new form:  b, d, g.  The pattern as such is preserved, or restored. Provided that the new series b, d, g does not become confused with an old series b, d, g of distinct historical antecedents.  If there is no such older series, the creation of a b, d, g series causes no difficulties.  If there is, the old patterning of sounds can be kept intact only by shifting the old b, d, g sounds in some way.  They may become aspirated to bh, dh, gh or spirantized or nasalized or they may develop any other peculiarity that keeps them intact as a series and serves to differentiate them from other series.  And this sort of shifting about without loss of pattern, or with a minimum loss of it, is probably the most important tendency in the history of speech sounds.  Phonetic leveling and “splitting” counteract it to some extent but, on the whole, it remains the central unconscious regulator of the course and speed of sound changes.