There’s much written on Mozart’s influence on Beethoven. Many have pointed out how the slow movement of Beethoven’s “Pathétique” Sonata in C minor sounds exactly like the beginning of a melody that occurs in the slow movement of Mozart’s Sonata in C minor, K. 457. Others have pointed out that Beethoven’s ‘Ode to Joy’ theme in his 9th Symphony is very similar to some of the melodic material in Mozart’s K. 222. But I think I’ve made a new discovery.

Listen to the opening of Mozart’s ‘Dissonance’ String Quartet in C Major, K. 465: https://youtu.be/kcfDxgfHs64

Now listen to the opening of Beethoven’s ‘Waldstein’ Sonata: https://www.youtube.com/watch?v=lbblMw6k1cU

Even though the tempo and meter and melodic material are totally transformed with the Beethoven, the harmonic outline is the same: A "I" chord in a C major piece with the root repeated in the bass, shifting into a V2 of V (G Major) and then to a V6, followed by a sequential restatement of the opening down in the subtonic B-flat, making a descending chromatic bassline from C. That's not all! The introduction of the Mozart ends on an open cadence awaiting C Major, just as does theme 1 of the Beethoven.

Is it far-fetched that Beethoven was at the piano playing Mozart’s string quartet, started toying with it in a fast 4/4, and then thought it would suit the opening of a new piece? I think not.

Just to stir your curiosity, I’d like to point out the opening of Mozart’s C minor Fantasy K. 475 is also very similar in its harmonic outline: https://www.youtube.com/watch?v=Ui9pyxdVX6Y

Ludwig van Beethoven Piano Sonata No. 21 in C Major (Waldstein), Op. 53