The answer to the great question: What is the mystery chord?
We can all hear the chord and recognise the sound, but in order to discern what was actually played we need to consider the instruments used, how they are played and recorded, and what notes are likely to have been played in the context.
There are also anecdotal recollections from the musicians involved and visual records of performances available for reference.
...and then there is discrete fourier transform - a mathematical process which determines the audio frequencies present in a recorded signal.
The following discussion presents the most probable voicings based on the empirical evidence of sound and Beatle-lore and then uses spectrum analysis to verify the assumptions.
While a purely scientific method may appear decisive it won't actually tell us what notes are being played on a musical instrument. Effective interpretation of the audio spectrum requires ancillary knowledge: the physics of sound, the nature and significance of overtones, the harmonic series, musical intervals, pitch perception, tuning.
Spectrum analysis for this document is provided by Andy Robinson's excellent Transcribe! software. The program translates the frequencies (numbers) into a graphic depiction of spikes aligned with notes on a piano keyboard. Below is the chord in question in glorious remastered mono.
Before analysing this tiny section note that in the entire recording there are two important considerations:
Most of the energy of the fundamental frequencies of the notes in the bass part are effectively mixed out. Nevertheless the brain still perceives the low notes when the overtones are heard.
The stereo mix provides some separation of the various instruments contributing to the sound of the opening chord and is therefore the basis of this analysis. Live concert and BBC performances are additional references. The mono mix is of little use but the "LOVE" remix proves crucial.
George Harrison 12-string Electric Guitar plays Fadd9
F2 A2 F3 A3 C4 G4
The distinctive 'chime' comes from the cluster of F G A in the 4th octave. The close voicing occurs as a result of the 3rd and 4th-strings being doubled an octave higher - simplifying what would require a wide stretch on a regular six string guitar.
Paul McCartney Bass plays D2
The spectrum suggests D3 is played however a full set of overtones from the lower octave is present.
The black vertical lines indicating the harmonic series are generated by the Transcribe! software. They appear in the correct proportion to display the series relative to any point selected in the view - note that the grid is shifted sharp to align with the spikes suspected as being the out-of-tune Bass playing D2.Furthermore, the note sounds the same as the D-notes played throughout the song and Paul plays D2 in all the live concerts and BBC sessions.
John Lennon 6-string Acoustic Guitar plays Fadd9
F2 A2 F3 A3 C4 G4
This voicing is completely isolated in the "LOVE" remix using the phase cancellation or 'karaoke' technique. (Invert the phase of one channel then sum the two channels)
Spectrum analysis of the right channel of the original stereo mix reveals complete sets of overtones for additional notes D2 and G2 but no fundamental tones. These are from the Piano played by George Martin with the bottom again EQ'd out of the mix.
Additional research involved notch-filtering to locate upper harmonics and compare with the isolated Acoustic Guitar. The harmonic content of each instrument is different largely due to the respective striking/strumming point on the string - all of the harmonics are much stronger in the piano than in the guitar.
Combining this information with aural perception the Piano notes are likely:
This voicing of Gsus4 is musically appropriate since it creates clusters of F G A in the low octaves, doubles the Bass part and doubles the C for good measure.
By hitting the chord with the sustain pedal down all of the strings of the piano are able to vibrate in sympathy. These strings produce numerous audible pitches which correspond to the harmonics of the notes played.
Example: D3 will induce pitches of A4 and A5 respectively in the strings A2 and B2
The tones are in just tuning and clash with each other as well as with the equal temperament of the played notes creating an extremely rich sound which cannot be reproduced any other way.