Question: In the song "I'll Tell You", what chord is played during the opening melody?
About the article "I Want to Hold Your Hand", section "Musical structure".

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The song is in the key of G and opens on "I'll tell you" with a D-B, B-D melody note drop and rise over a I (G) chord.
Controversy exists over the landmark chord that Lennon stated McCartney hit on the piano while they were composing the song.
Marshall considers it is the minor vi (Em) chord (the third chord in the I-V7-vi (G-D7-Em) progression).
Everett is of the same opinion.
Pedler claims, however, that more surprising is the melody note drop from B to F# against a III7 (B7) chord on "understand".
Music theorists are divided over whether this chord is a iii (Bm), a B major, or a B7 or even a B5 power chord with no major or minor defining third.
The Song is in the key of E and begins (on "If you wear red tonight...") with a I–IV-ii7-V7 chord progression (E-A-F#m7-B7) in which the word "to"night"" (B melody note) appears as a "delicately haunting" 4th above the F#m7.
The melodic pitches of this first two bar phrase are repeated (with initial repetition of the G# melody note) on "remember what I said tonight," except that the B melody note on the second "to"night"" is now backed by a ♭VII (D) chord that shapes the B melody note into a more "luscious" 6th.
The chorus ("Yes it is, it's true.
Yes it is, it's true") involves a I (E chord)- III (G# chord)- IV (A chord)- I (E chord) progression in which the major III (G#) chord appears for the first time in the Song to propel the Plagal drop from IV (A) to the tonic I (E) chord. 
The song initially was composed in C, but was played in F on "Rubber Soul" (with a capo on the fifth fret).
The verse opens with an F major chord ("Michelle"- melody note C) then the second chord (on "ma-belle"- melody note D) is a Bm7 (on the original demo in C, the second chord is a F7#9).
McCartney called this second chord a "great ham-fisted jazz chord" that was taught to them by Jim Gretty who worked at Hessey's music shop in Whitechapel, central Liverpool and which George Harrison uses (as a G7#9) (see Dominant seventh sharp ninth chord) as the penultimate chord of his solo on "Till There Was You".
After the E6 (of "these are words"-) there follows an ascent involving different inversions of the D dim chord.
These progress from Adim on "go"- melody note F, bass note D; to B(C)dim on "to"- melody note A, bass note D; to Ddim on "ge..."- melody note B(C) bass note B; to Bdim on ...'ther..."- melody note A bass note B, till the dominant (V) chord (C major) is reached on "well"- melody note G bass note C. 
Brian Wilson has explained that some of the melody in "God Only Knows" was inspired by the 1965 song "You Didn't Have to Be So Nice" by the Lovin' Spoonful.
The song is reputed for its complexity, and is in the form of ABCBCDECBC'.
The verses make extensive use of inverted chords.
The first chord (D major/A) is a non-diatonic chord.
Every time the dominant chord (E major) occurs, it’s in its most "unstable" inversion, with the major 3rd or the 5th in the bass.
The entire verse progression sounds restless, insecure, and looking for a direction, which is representative of the lyrical themes.
With the line ""God only knows what I'd be without you"" the chord progression finally displays confidence, moving to a clear harmonic goal (A—E/G#—F#m7—E).
This has been cited by musicologists as a good example of how lyrical meaning can be supported and enhanced by a chord progression—along with the melody hook which also provides an example of "a sense of increasing melodic energy that comes by way of the gradually ascending line.
"The song is told from the point of view of a man or woman contemplating life after death to his/her lover, as Tony Asher describes, "'I'll love you till the sun burns out, then I'm gone,' ergo 'I'm gonna love you forever.'"
Wilson has said that he initially hated the opening line of the song as "it was too negative."
He eventually gave in after hearing the subsequent lyrics.
Asher describes his interpretation:"God Only Knows" was one of the first pop songs to use the word "God" in its title, a decision that Wilson and Asher agonized over, fearing it would not get airplay as a result.
As Brian's former wife Marilyn describes, "The first time I heard it, Brian played it for me at the piano.
And I went, 'Oh my God, he's talking about God in a record.'
It was pretty daring to me.
And it was another time I thought to myself, 'Oh, boy, he's really taking a chance.'
I thought it was almost too religious.
Too square.
At that time.
Yes, it was so great that he would say it and not be intimidated by what anybody else would think of the words or what he meant.
"Tony Asher also explains that he and Brian "had lengthy conversations during the writing of 'God Only Knows', because unless you were Kate Smith and you were singing 'God Bless America', no one thought you could say 'God' in a song.
No one had done it, and Brian didn't want to be the first person to try it.
He said, 'We'll just never get any air play.'
Isn't it amazing that we thought that?
But it worked.
"Wilson explains that although he feared putting the word 'God' in the title of the song, he eventually agreed to keep it.
He explains that he agreed to keep the word 'God' in the title firstly, "because God was a spiritual word," and secondly, because Brian and The Beach Boys would "be breaking ground."
The song is in the key of D and begins with a I-IV-V7-I-IV-V7-I chord progression: thus "You know" involves F#-D# melody notes against a I (D chord).
A point of interest is the raised A melody note against a D/F# chord on "name", "three" and "name".
A significant moment is the substitution of a #IVdim chord (G#dim) for the more standard II7 as part of the progression to V7 (A7 chord on "You "know" my name") and I (D chord after "number") that closes the verse.
The song is also notable for use of the 5th chord tone on the VII chord to produce extra dissonance.