Endless Summer pins the voice on a single D and lets the harmony change what it means. Over the home B-flat the D is the major third, the chord voiced as a B-flat with its third in the bass. When the floor moves to a C seven suspended, that same D becomes the ninth. When the floor steps to D minor, the held D turns into the root. One pitch, three jobs, the meaning shifting under a melody that holds still.
This is recoloring, not a withheld third the voice supplies. The third D rings in the bass of the B-flat-over-D chord, so the guitar is not hiding the third for the singer to add. The harmony slides underneath a fixed note and reassigns it, the same trick the album runs in other songs, here built on the one held D. The voice stays put and the chords do the work.
The same held note carries the song's sharpest move. The voice trades its D for a C-sharp in the same melodic slot, and that half-step turns the home chord over. D is the major third of B-flat, C-sharp is the D-flat enharmonic, the minor third. The home chord switches from B-flat major to B-flat minor on one sung step, the brightness flipping to shadow inside a single phrase.
Nothing else has to change for the flip to land. The bass holds B-flat, the floor stays put, and the whole quality of the chord depends on whether the voice sings D or C-sharp. The song keeps its harmony still and lets the third decide major or minor, the same contested-third idea the record returns to. Here it happens on one note bending a half-step.
The close rocks the home chord against its dominant. The guitar trades a B-flat-five with an F seven suspended, the one chord against the five chord, the dominant framing the song the same way the F-area intro did at the open. The chart shows that rock as B-flat to F, the bookend that frames the whole track. The motion stays plain and the two chords trade to the end.
The last sound is a bare B-flat-five, no third. After a song that spent its length deciding whether the third was a D or a C-sharp, major or minor, the close declines to pick either. It ends on the open power chord, the third left out, the major-or-minor question held open. The dominant frames the song and the bare home chord finishes it third-less.