Time signature - 4/4
The piece begins in the key of C major, due to the absence of sharps or flats in the key signature (and the small clue given by the title - "Invention in C major"). Immediately, a theme is introduced that contains two separate motifs. The theme begins on the first C of bar one and ends on the first note of bar 2. The first motif contains 7 semiquavers, beginning on C, followed by a quaver (G). The second motif, beginning on the C, includes 4 quavers followed by a D semiquaver that is played with staccato. The use of this staccato works well to create a gap, emphasising the length of the theme. The second note of this motif has an upper mordent ornament, meaning that the B should be played as a rapid B-C-B. The use of this mordent in this motif is only repeated once in the next bar and not throughout the piece but considering it is just ornamentation, the style of the motif is still the same.
The first motif appears as an exact imitation an octave lower in the bass stave before the theme is finished. An exact imitation being a repetition of an idea with the same rhythmic structure and melodic intervals - usually played by a different voice. It doesn't necessarily have to begin on the same note, however. In this case, the entire theme (except the final note at the start of bar 3) is repeated in the second bar, beginning on the G, but transposed up a perfect 5th. The notes are different but the intervals between them remain exactly the same as the original motif. This can therefore be described as another exact imitation.
The first half of bar 2 could be considered as being in the key of G major because of the G in the bass and the D (perfect 5th above G) in the treble line. In the second half of the bar there is an F rather than an F#, which means that it is diatonic to C major and also implies a G7 chord. With the addition of the G and B in the final few notes of the bar, all the important tonal qualities of G7 are present which makes a perfect cadence (V-I) into C major at the beginning of bar 3. The bass melody in bar 3 also includes an imitation of the first motif but transposed up a 5th.
The treble in bar 3 begins with a diatonic melodic inversion of the first motif but played a 13th up. The motif is played 4 times to the end of bar 4 in a descending sequence. (Not all a 13th up). An inversion is a form of imitation where the notes are played in the opposite direction to the original part. For example, if a note in the original part descended by a 5th, the corresponding note in the imitation would ascend by a 5th. The fact that it is a diatonic melodic inversion rather than a literal melodic inversion means that the key signature is taken into account and every note must remain diatonic even if the interval isn't exactly the same. So for example, in the key of G major, a G might descend into an F# in one voice (a minor second interval) whilst an E might ascend into an F# in the other (a major second interval.) These 2 both include 3rd intervals except one must be altered in order to reach the diatonic note. If it was a literal inversion, the G might still descend to an F# but the E would need to ascend to an F, meaning both intervals were identical as a minor 3rd.
In bars 3 and 4 there is another descending sequence in the bass based on the second motif that is played 3 times. It is an augmented version of the motif. This means that the note lengths are doubled. So in this case, the original motif uses semiquavers whereas the augmentation uses quavers. The opposite would be diminution where semiquavers would be halved to become demisemiquavers. Bar 4 includes a literal melodic inversion of the first motif, which introduces an F#, suggesting that the piece is modulating into G major. The F# combined with the C in the bass could be seen as the major 3rd and the minor 7th of the D7 chord. Although there isn't actually a D present, the two important tonal qualities are still there so this could imply a secondary dominant in C major. D7 is the V chord of G major so this fits well.
The bass at the beginning of bar 5 is an exact imitation of the first motif although it begins on a D instead of a C. It is then followed by an augmentation of this motif leading into bar 6. The notes at the end are played in a different order but the rhythm is still doubled exactly so it can still be considered an augmentation. The bass at the end of bar 6 has a dotted quaver rhythm, which is similar to the treble in bar 5 except without the lower mordent. Half way into bar 5 there is a diatonic inversion of the first motif that begins on a B rather than a C. The end of this imitation repeats ascending alternating 3rds into the bar 6.
Bars 7 and 8 are similar to bars 1 and 2 except the bass and the treble (or the left hand and right hand on the piano) have swapped parts. This means that the treble begins imitating the bass. The bass in bars 7 and 8 is an exact imitation of the treble line in bars 1 and 2 except now it is in the key of G major, with each note diatonic to G major. The treble line now includes rests, similar to the bass of Bars 1 and 2 except not imitated exactly.
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