How are diatonic chord progressions different from chromatic chord progressions?

In music, the primary difference between diatonic and chromatic chord progressions lies in their relationship to the established key of a composition.1

• Diatonic Chord Progressions: These are based strictly on the seven chords derived from the specific scale of the key in which a piece is written. Because they utilize only the notes inherent to that scale, diatonic progressions provide a stable, predictable, and clear harmonic framework.1
• Chromatic Chord Progressions: These incorporate chords or individual notes that fall outside the primary key. By introducing these "outside" tones, chromaticism adds harmonic complexity, increased tension, and a sense of surprise or unanticipated movement to the music.12

While most music composed after 1600 remains fundamentally diatonic, composers often use chromatic tones to enhance expression. From the mid-19th century onward, composers such as Richard Wagner and Franz Liszt increasingly favored chromaticism to create more complex harmonic structures.2