A couple of years ago, I saw Paul McCartney in concert. And I have to say … it was awesome. He played dozens of songs over a couple of hours, making it the longest concert I’ve ever attended. And of course, that only makes sense—because he’s got about 10 trizillion excellent songs to choose from.

His set list included Beatles hits, Wings songs, and solo works covering all three “periods” of his songwriting career. But one of the best moments was when he performed the song “C Moon” (which was written in 1972).

This was already a great song, but McCartney’s description of the lyrics made it even cooler. The words play on the old 1950’s phrase “L7,” which is slang for the term “square”—meaning unhip loser. McCartney figured that the opposite of a square is someone who’s cool … or a “circle.” So in this song, he sings about “C Moon,” which represents a cool circle—or the opposite of an “L7” square. Get it?

As he told this story at the concert, McCartney laughed and said that squares and circles are an education in rock and roll. And as it turns out … they are. So now that we know about all the intervals in music, let’s see how Paul McCartney uses them in this classic hit: