From This Moment On was written by Cole Porter in 1950. The song follows an AABC form and was originally in the key of Ab. This is an analysis of the harmony in From This Moment On and an explanation of the concepts used in the analysis. The image above shows my full analysis, which I will explore in more detail below. You can refer to the conventions page to explore the analytic concepts used or get the PDF for free.
The A section is centered around the relative minor vi chord (F minor if in the key of Ab). The first four bars are simply a i-vi-ii-V in relation to the vi. This is followed by a ii-vi before going to a ii-V-I of IV. This is followed by a bVII leading to the I before a ii-V of vi turnaround. The second ending is a ii-V of IV leading to the B section.
The B section goes from IV to bVII to I to iii to VI before hanging out on essentially an extended V. There is a brief ii-V of V before going back to V and then a V of vi leading into the C section.
The C section (shown in Page 2) is identical to A until the final 6 measures. Here, there is a diatonic cycle of fourths beginning with iii and going to VI, II, V, I.