The acknowledgment of a full scale with its absolute tonality as a unit
of thought led onward toward a still more momentous possibility. If a
cluster of tones might be thus unified about a single centre, it followed
that by altering one or two of them the centre could be changed, and,
since the alteration might be smoothly introduced, a fresh sort of progress
might be worked out, whereby, besides passing from chord to chord, the
harmony might move from scale to scale or from key to key without losing
continuity and with a decided gain in expressional and structural value.
Though the modern theory of free modulation in all directions was still impossible,
owing to current methods of tuning, yet definite advances toward
it now began.
The increasing study of popular music tended constantly to
alter theory in other ways. It brought to the front the question
of 'form'—the lay-out of movements in sections or phrases,
each with some completeness in itself, and all with some definite
relation of length to the others. This was directly opposed to
the genius of contrapuntal structure as usually conceived, and
was probably resisted as vulgar and mechanical. But its utility
as a principle of organization gradually became clear, especially
in lighter composition. In dances and popular songs it was
indispensable. The problem was to use it without sacrificing
the general continuity and unity of the whole movement. This
higher unity, it was already seen, could be secured only through
resemblances or contrasts of melodic and harmonic design between
successive strophes.
Popular music also brought into view the artistic possibilities of the solo. Not only might a melody have beauty in itself and be highly expressive, but, as the doctrine of tonality unfolded, it appeared that a melody carried within itself the implication of an harmonic sequence, which was its essence. This threw a new light upon accompaniments, showing that the primary office of the latter was not to supply further independent melody, but to declare the chord-foundation that the melody implied.
This chord-foundation, it was found, could be outlined by means of a
continuous bass upon which the desired chord-series could be built up in
accordance with a few simple rules. To suggest a whole effect, then, it
was necessary only to indicate the intended melody and the appropriate
bass with some conventional signs attached, and the proper harmony could
be supplied at sight. This was the principle of the 'basso continuo' or
'thorough-bass' that came into use immediately after 1600.
