Robert Johnson recorded "Love In Vain" during his final session in June 1937. The song resembles Leroy Carr's 1935 tune "When The Sun Goes Down" in several significant ways. Though the lyrics of the two songs are different, the melody, harmonic structure and tempo are very similar. (Both "When The Sun Goes Down" and "Love In Vain" follow a twelve bar blues form.)
"When The Sun Goes Down" is in Bb, and aside from a brief introduction and coda, repeats a twelve bar pattern with the following harmonic structure (all 7s indicate minor sevenths above the bass; blank measures indicate continuation of the harmony from the previous measure):
I | V(b13)* | I | I7 | IV7 | | I - V(7) | I | V(7)! | | I - V7 | I |
* Once, at 1:50-1:53, a I chord is used here instead.
! Except for the chord beginning at 2:08, the chordal third (A) is omitted here.
In terms of melody and harmony, the two most characteristic moments here occur in measures 5 and 9. In measure 5, an F (scale degree 5) is repeatedly emphasized in the melody against an Eb7 chord. In measure 9, the descending line F-D-Bb-G (scale degrees 5-3-1-6) is heard in the vocals against what is most often the chord F-C-Eb (F7, without a chordal third).
I will call the tonic pitch of Johnson's "Love In Vain" Ab. After an introduction, the song repeats the following harmonic structure:
I(7) | I7 | | | IV | | I7 - V7 | I | ? | V7* | I(7) | (I - V7) |
* This chord always appears in first inversion (Eb7/G).
Notable differences in harmonic structure from Carr's "When The Sun Goes Down" include the use of a "turnaround" in the last bar (as in 0:41-0:43), the lack of a chordal seventh in mm. 5-6 (as in 0:22-0:27) and the recurring, descending chromatic line Gb-F-Fb-Eb [spanning scale degrees (b)7-5] in measure 11 (as in 0:38-0:41).
The most striking deviation from "When The Sun Goes Down" occurs in measure 9 (where the question mark appears above; see 0:32-0:36). In measure 9, Johnson consistently plays the following chord (from low to high): F-Bb-F-Ab (scale degrees 6-2-6-1). This chord acts as a pre-dominant, employing stepwise voice leading toward the Eb/G chord in measure 10 (the chordal seventh, Db, is not approached linearly). The specific voice leading used in mm. 9-10 is shown below.
m. 9 - m. 10
Ab - G (s.d. 8-7)
F - Eb (s.d. 6-5)
Bb - Bb (s.d. 2)
F - G (s.d. 6-7)
This pre-dominant harmony in measure 9 could be called "ii7 in second inversion with a missing chordal third," but that seems rather unnecessary, given that it seems to arise primarily from stepwise voice leading elaborating V. The inclusion of this chord creates a beautiful and particularly memorable moment. Using scale degree 6 as the bass note, it is almost as if Johnson has converted part of Carr's melody into harmony, as the final note of the melodic line from measure 9 of "When The Sun Goes Down" (scale degree 6; see 0:32-0:35 of the Carr) does not appear in Johnson's vocal at the comparable point in "Love In Vain." Instead, scale degree 6 is used as the bass note for the entire measure.