Question: What does the refrain in Girl (John Lennon's ballad on Rubber Soul) have in common with classics like Blue Moon and Heart and Soul, and with many hits from the 1950's such as Silhouettes on the Chaise, Teen Angel, All I Have to Do Is Dream, Duke of Earl, Earth Angel, Donna and Stay? Every Breath You Take by the Police and countless other songs display the same characteristic.

Answer: They all use a chord progression called "the ice cream changes." During the 1950's, this sequence was so common that some musicians still refer to it as "the 50's progression." No one knows for sure why it was dubbed "ice cream changes," but a consensus has formed around the idea that these chords were simple, sweet and familiar – like ice cream.

Technically, the progression is described as I-vi-IV-V, as in C-Am-F-G or G-Em-C-D. Lots of minor variations are possible, such as not going to minor in the second chord (I-VI-IV-V), or by using the relative minor for IV in the third chord (ii). In the latter case, the sequence in C would be C-Am-Dm-G. Also, the final V chord was frequently played as V7, forcing a resolution back to the tonic. This resolution to tonic, technically called a cadence, made the last chords.

The simplicity factor in the "ice cream changes" comes from their cyclical or circular pattern. They could be repeated interminably, until the song ran out of words. Not only were the chords not particularly hard on the guitar or piano (in a key such as C), but it was difficult to lose your place or fail to make a chord change correctly.

So pervasive was this chord progression in the 1950's that the average garage band guitarist of that time could fake his (or her) way through most of the top 40 songs, using just this sequence and one other: the 12-bar blues. The latter was imported into rock and roll from blues and boogie-woogie. Some students of the era will even declare, with permissible hyperbole, that prior to the Beatles, pop music had only two songs: the Blue Moon pattern and the blues pattern, exemplified by Chuck Berry's Johnny B. Goode.

The innovative use of different progressions in Beatles' songs is one of the strongest reasons for their immense success early on, and for their continued prominence and influence on the music scene for many years afterwards. Of course, the Beatles also took recourse to the "ice cream changes" at different times, especially when they covered songs by other artists, most notably doo-wop singers. But unlike their musical predecessors, the Beatles used the progression sparingly. It did not become a cliché in their hands. Whereas the Beatles' music still sounds fresh decades after they disbanded, the "ice cream change" songs sound nostalgic and, well, like relics from a distant era.