How does Eb connect musically to C major chord progression like in this song?

Question

Im learning basic music theory and chord progression then I stumbled upon this song (You are my song by Martin Nievera) :

From my understanding, the first 2 lines are simply C major chord progressions with pattern: I vi IV V .

But the 3rd and 4th lines are confusing. How did Eb sounded proper in the sequence from the C major pattern? My guess is it because its part of C minor scale? What makes it more confusing is at the end, the C major scale connects again smoothly. Eb is not even part of C major's median and dominant scale degrees (Em and G). What could be possibly happening in this one and what part of music theory this concept belongs? Thanks!

Answer 1

A C major to Eb major chord succession is very common in classical style music. It's termed a "chromatic mediant." The "strict" definition is two major or two minor chords having roots a third apart. It's a smooth transition in that there is a common tone between the chords. C major and Eb major share the note G. These provide a nice "pivot chord" for modulations to fairly remote keys.

In classical (Common Practice Period, about 1600-1900 or 2021 even) theory, chords are not made from scales. Scales are orderings of commonly played notes from a given key and the chords belong to a key. Some pop and jazz musicians prefer to think of scales plus chords but CPP more often is constructed in terms of keys and key relationships.

The chromatic mediant generally signals a (perhaps local) key change but could be used just for decoration. There are chromatic chords that do not necessarily signal key changes too. (Neapolitan Sixths, Augmented Sixths, secondary dominants....)

Answer 2

The first two lines are in the key of C major, then then the song modulates to Eb major. The modulation is unprepared: there is nothing at the end of the second line that makes us anticipate the change. To me the change seemed quite sudden, but there are indeed two factors that glue the two parts togeter.

• as ttw and Bennyboy1973 wrote, there is some relationship between the keys of C and Eb.
• the chord progression from the first part is exactly repeated transposed in the second part, and large part of the vocal melody is transposed as well. This quickly establishes the new key (of Eb) and makes us feel "at home", rather than thrown off balance by an unexpected change.

Answer 3

Given that the song shifts to E♭ major, and E♭ major shares the same key signature as C minor, I don't think it would be unreasonable to draw a connection.

That being said, motion by 3rds, either minor OR major, is pretty common, because of notes shared between them.

C-A major (share E)

C-A♭ major (share C)

C-E major (share E)

C-E♭ major (share G)

Note that a 3rd means two letter names apart: C (B) A. C and A are a 3rd apart. The above are some examples.

You can safely move to some very exotic and interesting chords by following just one rule: make sure that any 2 consecutive chords share at least one note.

C major (CEG) -->

E major (EG#B) (shares E) --> a 3rd from C

c# minor (C#EG#) (shares E) --> a 3rd from E

f minor (FA♭C) (shares A♭ = G#) --> a 3rd from c# respelled as D♭

F7 (FACE♭) (shares F and C) --> making V7 of B♭

B♭ (B♭DF) (shares F) end with a V-I in a new key

Moving up or down a 3rd C-> A, A♭, E, E♭ is a very common way of using this technique, but there are VERY many other possibilities.