e two voices play about, with
figures clearly derived from the motive, until we reach, in measures
three and four, a systematic downward transposition of the material.
Such transpositions or shiftings up or down in pitch are called
_Sequences_. They are very frequent in all polyphonic composition,
give a strong sense of unity to melodic progression and are generally
carried out in groups of three, _i.e._, the original figure and two
repetitions. After the sequence the music naturally works toward the
most nearly related key (the dominant) and in the seventh measure
reaches in that key its first objective. These Inventions of Bach, as
well as the Dance forms soon to be studied, are almost invariably in
what is known as _Two-part_ form, _i.e._, the music consists of two
main divisions, clearly marked off by cadences[37]; the first of which
modulates to the dominant or some related key while the second part,
starting in this key, works back to a final close in the home key. In
Inventions it early became customary in the second part to begin with
the same motive as the first--but in the _opposite_ voice. Thus we
see, in the Invention now being discussed, that the seventh measure
begins with the original motive in the bass which, in turn, is
imitated by the Soprano--a process just the reverse of that in the
opening measures.

[Footnote 36: The best edition is that by Busoni, published by
Breitkopf and Haertel.]

[Footnote 37: This technical term as well as others will later be more
fully explained.]

[Music]

In pieces in this Two-part form the second portion is generally longer
than the first; for the composer, by the time he has reached this
second part, may consider the material sufficiently familiar to be
expanded and varied by excursions into more remote keys, and by more
intricate manipulations of the chief motive. In measure 11 we find a
modulation to D minor and then, after some free treatment of the
motive, we reach--in measure 15--a cadence in A minor. A long
sequential passage brings us, through a modulation to the subdominant
key of F major (in measures 18 and 19), to a strong closing cadence in
the home key. It should be noticed that in this Invention and in some
of the dance forms there is shown a strong leaning towards a
tripartite division of the material as is indicated by the _three_
cadences in measures 7, 15 and 22. Since, however, the middle part is
lacking in any strong _contrast_--which is such a