A major third is a musical interval that spans four half steps, or semitones, between two notes. For example, if you start on the note C and move up to E, you have created a major third. This interval is commonly found in major chords, which consist of the root note, the major third, and the perfect fifth. In Western music, major thirds are essential for creating harmony and melody. They contribute to the bright and happy sound often associated with major keys. Understanding major thirds helps musicians build chords and progressions that evoke specific emotions in their compositions.