 The song was originally written on acoustic guitar in the key of C major . The recorded version is approximately in B @-@ flat major ; owing to manipulation of the recording speed , the finished version is not in standard pitch ( some , for instance consider that the tonic is A ) . The introduction was played by McCartney on a Mellotron , and involves a I – ii – I – ♭ VII – IV progression . The vocals enter with the chorus instead of a verse . In fact we are not " taken down " to the tonic key , but to " non @-@ diatonic chords and secondary dominants " combining with " chromatic melodic tension intensified through outrageous harmonisation and root movement " . The phrase " to Strawberry " for example begins with a somewhat dissonant G melody note against a prevailing F minor key , then uses the semi @-@ tone dissonance B ♭ and B notes ( the natural and sharpened 11th degrees against the Fm chord ) until the consonant F note is reached on " Fields " . The same series of mostly dissonant melody notes cover the phrase " nothing is real " against the prevailing F # 7 chord ( in A key ) . 
