What is the relationship between major and minor chords within a key?

Major and minor chords within a key are related in that they are both built from the same set of notes, known as the key signature. Major chords are built on the 1st, 4th, and 5th notes of the key, while minor chords are built on the 2nd, 3rd, and 6th notes. This relationship creates a harmonious and cohesive sound within a musical piece.