concerto form may be regarded as an
expansion of the aria form to a scale worthy of the larger and purely
instrumental forces employed, and so rendered capable of absorbing large
polyphonic and other types of structure incompatible with the lyric idea
of the aria. The _da capo_ form, by which the aria had attained its full
dimensions through the addition of a second strain in foreign keys
followed by the original strain _da capo_, was absorbed by the
polyphonic concerto on an enormous scale, both in first movements and
finales (see Bach's Klavier concerto in E, Violin concerto in E, first
movement), while for slow movements the _ground bass_ (see VARIATIONS),
diversified by changes of key (Klavier concerto in D minor), the more
melodic types of binary form, sometimes with the repeats ornamentally
varied or inverted (Concerto for 3 klaviers in D minor, Concerto for
klavier, flute and violin in A minor), and in finales the _rondo_ form
(Violin concerto in E major, Klavier concerto in F minor) and the binary
form (3rd Brandenburg concerto) may be found.

When conceptions of musical form changed and the modern sonata style
arose, the peculiar conditions of the concerto gave rise to problems the
difficulty of which only the highest classical intellects could
appreciate or solve. The number and contrast of the themes necessary to
work out a first movement of a sonata are far too great to be contained
within the single musical sentence of Bach's and Handel's ritornello,
even when it is as long as the thirty bars of Bach's Italian concerto (a
work in which every essential of the polyphonic concerto is reproduced
on the harpsichord by means of the contrasts between its full register
on the lower of its two keyboards and its solo stops on both). Bach's
sons had taken shrewd steps in forming the new style; and Mozart, as a
boy, modelled himself closely on Johann Christian Bach, and by the time
he was twenty was able to write concerto ritornellos that gave the
orchestra admirable opportunity for asserting its character and resource
in the statement in charmingly epigrammatic style of some five or six
sharply contrasted themes, afterwards to be worked out with additions by
the solo with the orchestra's co-operation and intervention. As the
scale of the works increases the problem becomes very difficult, because
the alternation between solo and tutti easily produces a sectional type
of structure incompatible with the high degree of o