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 and is essential in Western music theory. In terms of sound, a major third has a bright and happy quality. It is often used in melodies and harmonies to create a sense of resolution and stability. Understanding major thirds is fundamental for musicians when constructing chords and composing music.